Update some docs for some nits found (#286)

* A few small doc clarifications

First of all, remove reference to HP as an OpenStack provider and
replace with some other examples of folks who are actually running
OpenStack Public Clouds.

Then add a few notes clarifying tenant/project usage and the fact that
domain applies to both.

* Change my-openstack.com to openstack.example.com

RFC 2606 defines example.com as a reserved domain to be used in tests
and examples. It is guaranteed to not be actually resolvable. Update
the uses of my-openstack.com to use example.com instead.

  https://tools.ietf.org/html/rfc2606

* Change "cluster" to "cloud"

The general term for an installation of OpenStack is cloud, at least
when talking to end users.

* Address review comments
diff --git a/openstack/compute/v2/extensions/keypairs/results.go b/openstack/compute/v2/extensions/keypairs/results.go
index f4d8d35..4c785a2 100644
--- a/openstack/compute/v2/extensions/keypairs/results.go
+++ b/openstack/compute/v2/extensions/keypairs/results.go
@@ -5,7 +5,7 @@
 	"github.com/gophercloud/gophercloud/pagination"
 )
 
-// KeyPair is an SSH key known to the OpenStack cluster that is available to be injected into
+// KeyPair is an SSH key known to the OpenStack Cloud that is available to be injected into
 // servers.
 type KeyPair struct {
 	// Name is used to refer to this keypair from other services within this region.
diff --git a/openstack/db/v1/instances/testing/fixtures.go b/openstack/db/v1/instances/testing/fixtures.go
index 79ee9c3..9347ee1 100644
--- a/openstack/db/v1/instances/testing/fixtures.go
+++ b/openstack/db/v1/instances/testing/fixtures.go
@@ -27,22 +27,22 @@
     "id": "1",
     "links": [
       {
-        "href": "https://my-openstack.com/v1.0/1234/flavors/1",
+        "href": "https://openstack.example.com/v1.0/1234/flavors/1",
         "rel": "self"
       },
       {
-        "href": "https://my-openstack.com/v1.0/1234/flavors/1",
+        "href": "https://openstack.example.com/v1.0/1234/flavors/1",
         "rel": "bookmark"
       }
     ]
   },
   "links": [
     {
-      "href": "https://my-openstack.com/v1.0/1234/instances/1",
+      "href": "https://openstack.example.com/v1.0/1234/instances/1",
       "rel": "self"
     }
   ],
-  "hostname": "e09ad9a3f73309469cf1f43d11e79549caf9acf2.my-openstack.com",
+  "hostname": "e09ad9a3f73309469cf1f43d11e79549caf9acf2.openstack.example.com",
   "id": "{instanceID}",
   "name": "json_rack_instance",
   "status": "BUILD",
@@ -114,14 +114,14 @@
 	Flavor: instances.Flavor{
 		ID: "1",
 		Links: []gophercloud.Link{
-			{Href: "https://my-openstack.com/v1.0/1234/flavors/1", Rel: "self"},
-			{Href: "https://my-openstack.com/v1.0/1234/flavors/1", Rel: "bookmark"},
+			{Href: "https://openstack.example.com/v1.0/1234/flavors/1", Rel: "self"},
+			{Href: "https://openstack.example.com/v1.0/1234/flavors/1", Rel: "bookmark"},
 		},
 	},
-	Hostname: "e09ad9a3f73309469cf1f43d11e79549caf9acf2.my-openstack.com",
+	Hostname: "e09ad9a3f73309469cf1f43d11e79549caf9acf2.openstack.example.com",
 	ID:       instanceID,
 	Links: []gophercloud.Link{
-		{Href: "https://my-openstack.com/v1.0/1234/instances/1", Rel: "self"},
+		{Href: "https://openstack.example.com/v1.0/1234/instances/1", Rel: "self"},
 	},
 	Name:   "json_rack_instance",
 	Status: "BUILD",