Run tempest tests with testr

Previously the rpc_tempest_gate.sh script used nose to run the tempest
tests. This patch switches to testr, which seems to be the community
consensus.

The primary reason for this is that test skips exceptions are
interpreted as skips not errors.

A secondary benefit is that testr revolves around test lists so we can
fine tune the test list without modifying tempest code.

Related:
  * XML tests failing: https://github.com/rcbops/ansible-lxc-rpc/issues/653
  * Tempest Testing: https://github.com/rcbops/ansible-lxc-rpc/issues/472