Sync in the charm-helpers to use the new VolumeAPIContext object
in order to determine the volume catalog info to use in the
configuration file.
This is simply an alternative implementation for commit 5d92bc9f.
This will separate concerns for determining internal endpoints
and determining volume api versions.
Change-Id: I91009e1f9643f818b6f97898aa5d7c43e84684ed
Related-Bug: #1733566
465617c263