Use /var/tmp/packstack/{%Y%m%d-%H%M} as working dir name

Collect all packstack's logs in a subdirectory called packstack and use
a '%Y%m%d-%H%M' formated datetime instead of a uuid for folder names.

Fixes bz#888360

Change-Id: Ib5a3bcc74e7819c3c3ee43d2b790922fad358e40