VMware Datastore storage backend

Customers using a VMware environment with OpenStack should be able to
store their Glance images in VMware datastores. This is a first step to solve
the problem where Nova needs to copy the bits over the network
from Glance to the datastore when spawning an instance.
Also, this give the ability to provide some optimizations for specific
image formats in the future (fast cloning for example).

This patch contains a 'glance/store/vmware/' folder with the code
to manage the connection with vCenter or an ESX(i) host.
This code will go away as soon as it is merged to Olso:
see review https://review.openstack.org/#/c/65075/

The current implementation give this ability to specify the vCenter or
ESX(i) IP. In case of a vCenter IP, there is no optimization to reduce
the datapath (no host selected).
Consequently, it is recommended to specify an ESX IP if the ESX host
API endpoint is accessible from Glance.

docImpact
Implements bp vmware-datastore-storage-backend

Change-Id: I3837912e0d1614b9c31a689f71c2e34d453e2dc3