nova_keypair should fail if 'name' of key exists in keystore, but ssh hash value != public_key offered

if key name already exists on tenant, check when public_key is offered that it matches the existing key

Example: Here is the signature
 - nova_keypair: state=present 
                 login_username="admin"
                 login_password="password"
                 login_tenant_name="{{tenant.name}}"
                 auth_url="{{os_auth_url}}"
                 name="{{tenant_name}}_key"
                 public_key="{{ lookup('file', '~.ssh/' + tenant_name + '_key.pub') }}"

Suppose you created a cluster with a certain public key A and deleted the cluster but not the tenant.  You generate a new key B with the same name locally.  If you try to add the new key with the same name but different ssh hash nova_keypair will currently not issue any warning or error.  But you won't be able to login to the new vms.