Add keystoneclient CredentialsManager if missing

The keystone v3 client doesn't include the 'ec2' manager, even
though the EC2 API is included in the default pipeline.

This commit creates the ec2 manager manually if it's missing
(and necessary). It uses the same call that the client itself
should have used.

Change-Id: I83e7bfa04cde5093c10bf2bc27af5ec03da4b48e
Closes-bug: #1236326