Moved create_instance DNS attempt to post-prepare

Moved the actual attempt to add DNS to after the prepare call because of
two issues 1) instances that have failures on DNS end up in a condition
in which the container is unusable - there is no support to send a
prepare call otherwise and 2) DNS isn't required for any operations
after it's addition thus making it a gating operation for prepare makes
no sense because if there is a failed DNS request, retrying that
operation is much less complex than re-issuing a prepare call and
support teams can handle this by hand.

blueprint trove-taskmanager-send-prepare-before-dns

Change-Id: I51fa2115e3d52a85aa87970903922f25bae3bf9f