Adds Fixed IP support to os-floating-ips

This commit enables the ability to specify a fixed IP when associating a
floating IP to an instance. If a fixed IP is not specified, Nova will
attempt to associate the floating IP to the first detected fixed IP, as it
did prior to this patch.
diff --git a/openstack/compute/v2/extensions/floatingip/requests.go b/openstack/compute/v2/extensions/floatingip/requests.go
index 8abb72d..941dc3b 100644
--- a/openstack/compute/v2/extensions/floatingip/requests.go
+++ b/openstack/compute/v2/extensions/floatingip/requests.go
@@ -26,6 +26,18 @@
 	Pool string
 }
 
+// AssociateOpts specifies the required information to associate or disassociate a floating IP to an instance
+type AssociateOpts struct {
+	// ServerID is the UUID of the server
+	ServerID string
+
+	// FixedIP is an optional fixed IP address of the server
+	FixedIP string
+
+	// FloatingIP is the floating IP to associate with an instance
+	FloatingIP string
+}
+
 // ToFloatingIPCreateMap constructs a request body from CreateOpts.
 func (opts CreateOpts) ToFloatingIPCreateMap() (map[string]interface{}, error) {
 	if opts.Pool == "" {
@@ -35,6 +47,26 @@
 	return map[string]interface{}{"pool": opts.Pool}, nil
 }
 
+// ToAssociateMap constructs a request body from AssociateOpts.
+func (opts AssociateOpts) ToAssociateMap() (map[string]interface{}, error) {
+	if opts.ServerID == "" {
+		return nil, errors.New("Required field missing for floating IP association: ServerID")
+	}
+
+	if opts.FloatingIP == "" {
+		return nil, errors.New("Required field missing for floating IP association: FloatingIP")
+	}
+
+	associateInfo := map[string]interface{}{
+		"serverId":   opts.ServerID,
+		"floatingIp": opts.FloatingIP,
+		"fixedIp":    opts.FixedIP,
+	}
+
+	return associateInfo, nil
+
+}
+
 // Create requests the creation of a new floating IP
 func Create(client *gophercloud.ServiceClient, opts CreateOptsBuilder) CreateResult {
 	var res CreateResult
@@ -68,6 +100,7 @@
 // association / disassociation
 
 // Associate pairs an allocated floating IP with an instance
+// Deprecated. Use AssociateFloatingIP.
 func Associate(client *gophercloud.ServiceClient, serverId, fip string) AssociateResult {
 	var res AssociateResult
 
@@ -79,7 +112,33 @@
 	return res
 }
 
+// AssociateFloatingIP pairs an allocated floating IP with an instance.
+func AssociateFloatingIP(client *gophercloud.ServiceClient, opts AssociateOpts) AssociateResult {
+	var res AssociateResult
+
+	associateInfo, err := opts.ToAssociateMap()
+	if err != nil {
+		res.Err = err
+		return res
+	}
+
+	addFloatingIp := make(map[string]interface{})
+	addFloatingIp["address"] = associateInfo["floatingIp"].(string)
+
+	// fixedIp is not required
+	if associateInfo["fixedIp"] != "" {
+		addFloatingIp["fixed_address"] = associateInfo["fixedIp"].(string)
+	}
+
+	serverId := associateInfo["serverId"].(string)
+
+	reqBody := map[string]interface{}{"addFloatingIp": addFloatingIp}
+	_, res.Err = client.Post(associateURL(client, serverId), reqBody, nil, nil)
+	return res
+}
+
 // Disassociate decouples an allocated floating IP from an instance
+// Deprecated. Use DisassociateFloatingIP.
 func Disassociate(client *gophercloud.ServiceClient, serverId, fip string) DisassociateResult {
 	var res DisassociateResult
 
@@ -90,3 +149,23 @@
 	_, res.Err = client.Post(disassociateURL(client, serverId), reqBody, nil, nil)
 	return res
 }
+
+// DisassociateFloatingIP decouples an allocated floating IP from an instance
+func DisassociateFloatingIP(client *gophercloud.ServiceClient, opts AssociateOpts) DisassociateResult {
+	var res DisassociateResult
+
+	associateInfo, err := opts.ToAssociateMap()
+	if err != nil {
+		res.Err = err
+		return res
+	}
+
+	removeFloatingIp := make(map[string]interface{})
+	removeFloatingIp["address"] = associateInfo["floatingIp"].(string)
+	reqBody := map[string]interface{}{"removeFloatingIp": removeFloatingIp}
+
+	serverId := associateInfo["serverId"].(string)
+
+	_, res.Err = client.Post(disassociateURL(client, serverId), reqBody, nil, nil)
+	return res
+}