Add post install job after VM creation

After a VM or VIOC creation, a post install job can be triggered.
The job to trigger is based on the domain and defined in the
AWX_POST_INSTALL dictionary.

Admin users have the option to skip this post job if required.

The job is enqueued in RQ with a dependency on the creation job.
So it's only triggered after successful completion of the first one.

JIRA INFRA-769