schedule: strip out targets

If the user feeds in a yaml with targets, the worker will launch the job but
fail with

2013-09-03T11:18:34.333 CRITICAL:root:AssertionError: You cannot specify
targets in a config file when using the --lock option

Just strip them out before scheduling.  This eases my personal workflow where
I have a test I'm running manually against some prelocked machines but also
want to schedule it.

Signed-off-by: Sage Weil <sage@inktank.com>