Fix volume attach tests failing when using FIP as ssh method

When use floating as connect_method, tests:
test_attach_detach_volume[id-52e9045a-e90d-4c0d-9087-79d657faffff]
test_list_get_volume_attachments[id-7fa563fe-f0f7-43eb-9e22-a1ece036b513]
will fail due to that they are under the same class, so when these tests
setup, and using floating as connect_method only one floating IP was
created.We first associated it to instance created at test1 and then we
created another instance in test2 and tried to associate this IP again to
the new server, since the IP was already associated to instance created in
test1, error will happen when try to do this.

This patch fix this by delete the servers after first test finished.

Closes-Bug: #1686664

Change-Id: Ie3afb31bd0d3245760f799b2256228159b328619

tempest/api/compute/volumes/test_attach_volume.py

@@ -52,6 +52,7 @@ class AttachVolumeTestJSON(base.BaseV2ComputeTest):
             validatable=True,
             wait_until='ACTIVE',
             adminPass=self.image_ssh_password)
+        self.addCleanup(self.delete_server, server['id'])
         # Record addresses so that we can ssh later
         server['addresses'] = self.servers_client.list_addresses(
             server['id'])['addresses']