Add keystone v2 keypair methods

To get KeystoneClientV2 to work with StackUser resources, the
missing methods create_stack_domain_user_keypair and
delete_stack_domain_user_keypair have been added.

StackUser._create_keypair has also been modified to tolerate
having a missing id attribute, which is what the v2 API returns.

Change-Id: I9c19c1ca49f72ff1e760e849712c8c9d5fa7541f
Closes-Bug: #1372687
Merge "Add keystone v2 keypair methods"