Add a FindMixin class to help with getting resources by name

Note: this is largely a copy of apiclient.base.ManagerWithFind
      but without the inheritance and the find() method which
      does not clash with the Manager find() - now called findone()
(tests are in the folowing commit)

Change-Id: I332f542a1bf372e239f7f4e6a760e3bffebb641c