Include oslo.config=1.2.0a3 for trove guest VM

python dependencies were being installed
(including oslo.config) before we install
trove inside guest VM,therefore the
oslo.config>=1.2.0a3 was not picked
 up from trove requirements.txt.
The same has been addressed here.

Change-Id: I5047594827bcc3f9f2e0d909d6e9c24f57d4fd19
Fixes: bug #1215001