Fixed TenantID struct-tag in user/CommonOpts (#211)

diff --git a/openstack/identity/v2/users/requests.go b/openstack/identity/v2/users/requests.go
index ef77d39..37fcd38 100644
--- a/openstack/identity/v2/users/requests.go
+++ b/openstack/identity/v2/users/requests.go
@@ -21,7 +21,7 @@
 	Name     string `json:"name,omitempty"`
 	Username string `json:"username,omitempty"`
 	// The ID of the tenant to which you want to assign this user.
-	TenantID string `json:"tenant_id,omitempty"`
+	TenantID string `json:"tenantId,omitempty"`
 	// Indicates whether this user is enabled or not.
 	Enabled *bool `json:"enabled,omitempty"`
 	// The email address of this user.