Merge lp://qastaging/~fnordahl/charm-helpers/keystone_configure_api_version into lp://qastaging/charm-helpers

Charms need a way to change preferred-api-version of Keystone in a deployment to perform their tests for a Keystone v3 enabled deployment.

Example usage:
    def test_42_dummy(self):
        se_rels = [
            (self.keystone_sentry, 'ceph-radosgw:identity-service'),
        ]
        u.keystone_configure_api_version(se_rels, self, 3)
        client = u.authenticate_keystone(
            self.keystone_sentry.info['public-address'],
            'admin', 'password', api_version=3,
            user_domain_name='admin_domain',
            domain_name='admin_domain'
        )
        u.log.debug('Keystone domains: {}'.format(client.domains.list()))
        u.log.debug('OK')

Note: The use of @retry_on_exception decorator requires addition of 'core' to include-list in charm-helpers-tests.yaml on sync of charm-helpers.