Cap oslo.vmware to <0.6.0

In 0.6.0, oslo.vmware upped its minimum eventlet requirement to >=0.15.2.
With recent migration of devstack to install libraries from release,
pip's poor transitive dependency resolution is not picking this up and
causing version conflicts.  This caps the requirement to 0.5.x and allows
running with eventlet 0.13.0.

Change-Id: I1465197a9546153fd28e12abfb3bf127176cb2a9
Closes-bug: #1395814