Move oslo.vmware higher in requirements.txt

oslo.vmware's transitive dependency on eventlet >= 0.15.2 is only resolved
and enforced if it is listed before requirements.txt's >= 0.13.0.  This
moves it up in the list to avoid version conflicts when installing from
release in devstack.

Related-bug: #1395814
Change-Id: I7f06a030cd9f31f3741c1ea4c97a2be3b76b978c