testauth: add a mechanism to specify groups via username
    
    This adds a mechanism to control testauth's reported group
    memberships via the username passed. You can pass a username
    like 'guest:groups=foo,bar', and you will be logged in as user
    'guest' with (only) the group memberships 'foo' and 'bar'. This
    provides finer-grained options for testing group memberships
    beyond just setting a default group membership for all users.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>