feat: allow domain names or IDs in keystone connector (#3506)

OpenStack Keystone allows a user to authenticate against a domain. That
domain can be specified either as the domain ID or the domain name when
authenticating. The domain ID is a UUID or the special "default" domain
ID so key off of that when deciding what to submit to the keystone API.
Collapsed the code to share the domainKeystone struct by utilizing
omitempty to skip unset fields.

Signed-off-by: Doug Goldstein <cardoe@cardoe.com>