[identity] many keystone cli calls

Using the identity cookbook register provider to create users, grant roles, I noticed it used UUID for all the keystone parameters. This requires a separate call to the keystone api to query the UUID. The keystone client (and the new openstack client) also accept the user and tenant name instead of the UUID, and it will do the query within the api library. See: https://github.com/openstack/python-keystoneclient/blob/master/keystoneclient/v2_0/shell.py#L93 and
https://github.com/openstack/python-keystoneclient/blob/master/keystoneclient/v2_0/shell.py#L329 and
https://github.com/openstack/python-keystoneclient/blob/master/keystoneclient/v2_0/shell.py#L366.

By switching the logic to simply use the user and tenant name instead of looking up the UUID, it will save 1 cli call for create_user and 2 cli calls for grant_role without any change in current functionality. This savings will add up quickly for normal use cases, a reduction of at least 12+ cli calls for better overall conver performance.