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: I7cb79186028e405462b92a6ba2e37298d5ca0f9b
Implements: blueprint vmware-api