VMware: Fix memory leaks caused by caches

Using suds objects as keys for the DatastoreBrowser cache is incorrect
because they don't implement __eq__ and __hash__ for the VIM types.
This always results in cache miss and the cache grows with every spawn()
operation.

This patch fix this by using the 'value' property (which is string) of
the MoRef as key.

Closes-Bug: 1316433
(cherry picked from commit bceb3f96b06ff8a048598724494299cf111bcaf8)

Conflicts:

	nova/tests/virt/vmwareapi/test_imagecache.py
	nova/tests/virt/vmwareapi/test_vmops.py

Change-Id: I2bcaf87e733d51055566aee41bb0a7e254027ba9
Merge "VMware: Fix memory leaks caused by caches" into stable/icehouse