Ensure only pickle-able objects live in metadata

Conductor api has been added to InstanceMetadata object, but it cannot
be serialised. This breaks storing metadata in memcache, because the
information needs to be passed through pickle.

Since the field was not used outside of the constructor, it can be
removed to fix the problem.
Also add a test that pickles a sample InstanceMetadata to catch similar
failures in the future.

Fixes bug 1160900

Change-Id: I7e8cef5a131290eaa1402d48295026bcac6baff6
Merge "Ensure only pickle-able objects live in metadata"