Fix adding tenant to keystone user with no previous tenant assigned

Applying a keystone.user_present state on a user without a tenant currently raises the following exception:

     Comment: An exception occurred in this state: Traceback (most recent call last):
                File "/usr/lib/python2.7/site-packages/salt/state.py", line 1371, in call                                                 
                  **cdata['kwargs'])                                                                                                      
                File "/usr/lib/python2.7/site-packages/salt/states/keystone.py", line 141, in user_present                                
                  if tenant and user[name]['tenant_id'] != tenant_id:                                                                     
              KeyError: 'tenant_id'

This commit applies the required key check.