Fixes VIO collection

Extract common vROPS code from vmware and vio collectors.
Add unit test coverage.
Remove unused code.
Improved vROPS collection to ask for current values only
instead of a date range.
Reduces number of calls to vROPS by asking for all metrics
for all VDUs at once.

BUG 796

Change-Id: I00ecd70c6d25f7c8982cbc633a28ab6f1ceb8cd6
Signed-off-by: beierl <mbeierl@vmware.com>