packaging: removed --no-mem-check from engine-setup

If minimum memory requirements are not met:
- on interactive mode, stop, warn and ask if to abort
- on answer file mode don't stop, issue the warning in the log.
The assumption here is that users of the answer file are aware
of the requirements and had to read the manual to use this option.

Moved requirements check after process initialization
avoiding double execution of the check.

Change-Id: I7180589490cf74fcb402bd3b957ae8ae8479715d
Bug-Url: https://bugzilla.redhat.com/928386
Signed-off-by: Sandro Bonazzola <sbonazzo@redhat.com>