Allow --domain to be used for identity commands without lookup

Performing create, list, or set operations for users, groups, and projects
with the --domain option attempts to look up the domain for name to ID
conversion.  In the case of an environment using Keystone domains, it is
desired to allow a domain admin to perform these operations for objects in
their domain without allowing them to list or show domains. The current
behavior prevents the domain admin from performing these operations since
they will be forbidden to perform the underlying list_domains operation.

This patch makes the domain lookup error a soft failure, and falls back
to using the passed in domain argument directly as a domain ID in the
request that it sends to Keystone.

Change-Id: I5139097f8cedc53693f6f71297518917ac72e50a
Closes-Bug: #1378565