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 16ae06bf
which separates the concerns for volume endpoint context and
internal endpoint context.
Change-Id: I187bc8ef5644a59dc5a0b5f7ad66774812a78b78
Related-Bug: #1733566
d43e264d02