VMware driver: Fix for invalid datastore selection

Added filter to ignore datastores that are not mounted, in-accessible
and read-only for volume creation. Without this VMware's VC/ESX driver can
choose such datastore with higher invalid capacity value as returned by the
server for volume creation.

Also using parent's self.mox object in the test cases rather than creating
a new mox.Mox obj everytime.

Fixes bug: 1220217

Change-Id: If8319562ddc11fc31eb928b3807aead2b11b94bc
Merge "VMware driver: Fix for invalid datastore selection"