restapi: Validate Step Enum (#1092448)

Up until now, if user supplied invalid step-type, the server would fail.

This patch adds a validation that the step-type is valid

Bug-Url: http://bugzilla.redhat.com/1092448
Change-Id: I7768fb15b0f9c5924cab0f4e2238cf49d21eb655
Signed-off-by: Ori Liel <oliel@redhat.com>