Move VIM API client code 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 classes in
VMware drivers for making VIM API SOAP calls into OSLO.

Change-Id: I115e2d093c19051c507061b50d7ee1d1851fc412
Implements: blueprint vmware-api