More changes for creating vms manually with lock (no config)

More changes for creating vms manually with lock (no config)

Needed some more changes to allow for the case of creating vm's
manually with teuthology-lock instead of letting teuthology handle
it in internal.py with lock_machines(). Just some additional checks
to go to defaults when ctx.config is non-existent (causes an
attributeerror).

Signed-off-by: Sandon Van Ness <sandon@inktank.com>
Reviewed-by: Warren Usui <warren.usui@inktank.com>