restapi : Add the option to set custom ticket to a VM(#886840)

Let the user set a custom ticket for VM. If custom ticket is provided
by the user the api/backend must use the ticket provided by the
use and not generate a new ticket.

Change-Id: I06b48b5ad300cb607e62f461ca7a07c16c59bf4a
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=886840
Signed-off-by: Ravi Nori <rnori@redhat.com>