Move admin client initialization into stress-openstack

Running the stress test framework without any tempest.conf causes
an error. This is due to the fact that the admin client expects the
configuration file. This causes error while doing pep8 checks (see
https://review.openstack.org/#/c/56308/)

Change-Id: I69b3d83185d1b144474314402ad5449980d6ffbc
Fixes: bug #1251663
Merge "Move admin client initialization into stress-openstack"