Fix oslotest import error

The barbican-tempest-plugin gate has stopped working
after export DEVSTACK_GATE_TEMPEST_ALL_PLUGINS=1 was
removed in Ibef3f9a135f14727bf57c29e766f838d7da56c68
since oslotest is no longer installed.

This patch uses the tempest.lib base test instead
of using the oslotest base test.

Change-Id: Ifdf8b426c21e4b3a51f97cbc3d95eb842eb04515
2 files changed