When trying to compare two values that are non-numeric using the driver
filter, the filter function will give an error. This is not desirable as
it might be interesting to support comparatives with non-numeric values
provided by the filter objects (volume, host, etc). For example, the
following formula failed before the fix:
filter_function = '(volume.project_id == "bb212f09317a4f4a8952ef3f729c2551")'
Change-Id: Ifc9f2d7cf39c0e497e1ed6440d2be4cda6be12d9