Update server support for Locate to filter on arbitrary attributes

[PeterHamilton]

The Locate operation can filter results based off a list of attributes provided in the request. Currently, the PyKMIP server only filters on the Name attribute (see here). Add support for Locate filtering based off of any attribute.

I can take up this feature. I want to know what are the most used attribute to filter on to start with.

[PeterHamilton]

This is pretty much complete after #577.