Move utility methods in VMware drivers to OSLO

There is lot of common code between VMware drivers for nova and cinder;
for e.g., session management and VIM API invocation code. The new
VMware driver for glance and the telemetry agent for vSphere are also
planning to use this common code. This change moves the utility methods
for inventory traversal spec creation, managed object property retrieval
etc. defined in various drivers into OSLO.

Change-Id: Ibdd80188ffb0936468545ceec86b67162a111fff
Implements: blueprint vmware-api