Fix vmwareapi driver get_diagnostics calls

The vmware driver is calling vmops.get_info for both the get_info and
get_diagnostics APIs which is wrong since they should be returning
different information.  Given the hypervisor support matrix doesn't list
the vmware driver as supporting the nova diagnostics API, it seems this
was just an oversight.

This patch changes the driver code to call the vmops.get_diagnostics
method which raises NotImplementedError.  It also fixes the tests to
actually ensure covers get_diagnostics for both the ESX driver and the
VC driver.

Closes-Bug: #1237622

Change-Id: I0c9015f1f7d5dd219548196de479e87e878ea0fd
Merge "Fix vmwareapi driver get_diagnostics calls"