restapi: Enable eject of cdrom via REST

Modified parameter validation to allow for files witout set id.

Also needed to normalize representation of "EJECT" as empty string on
backend in case `null` is received from REST api.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1135970
Change-Id: Ia11068f319bcb5cf0ff1facedd9cb99ce4c9e5d0
Signed-off-by: Martin Betak <mbetak@redhat.com>