Check if shell in kwargs is actually true instead of just present

Previously, process.__check_for_problems only looked for the
existence of "shell" as a kwarg, but the service call explicitly
sets shell to False. See whether or not it's true before we
throw a runtime error.

Change-Id: I12d7e73b18b0e23335b5f481d0e06d302f5b1438
Bug-Url: http://bugizlla.redhat.com/show_bug.cgi?id=1010629
Signed-off-by: Ryan Barry <rbarry@redhat.com>
Signed-off-by: Fabian Deutsch <fabiand@fedoraproject.org>