Remove admin auth when getting the list of Neutron API extensions.

Currently _refresh_neutron_extensions_cache() always authenticates
with Neutron using the configured Neutron admin credentials because
it doesn't have access to the calling context.  But it's actually
always called when a valid context exists, and the call to get the
list of extensions is not itself an admin operation.  By passing
the context down we can avoid this additional authentication step.

Change-Id: I9e7d9c5e424a6b39c8f8ee1c79f5fd8f48ad81a2
Closes-Bug: 1250850
Merge "Remove admin auth when getting the list of Neutron API extensions."