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>