Fix setting of users default tenant

Keystone v2.0 api expects tenantId to define the default
tenant, not tenant_id (which is the EC2 credential tenant)

(cherry picked from commit b9b20622fbc015bbab4209456c6e1c018fe3d512)
Merge pull request #167 from dirkmueller/release/pebbles/master

Fix setting of users default tenant