Image sync fails on multi-region shared keystone

When using a multi-region shared keystone, glance-simplestrams-sync fails to sync the image due to the fact that it tries to upload the image to the wrong region.

The same keystone offers endpoints for multiple regions. This charm calls into simplestreams.sstream-mirror-glance, but does not set the --region flag.

It seems that without it, sstream-mirror-glance will try to use whichever region it finds first in the keystone session that gets created. This is in spite of the fact that OS_REGION_NAME is set by this charm.

The fix is two part:

* https://code.launchpad.net/~gabriel-samfira/simplestreams/+git/simplestreams/+merge/452267
* and setting the --region flag when calling the binary (patch attached)

A PR with the attached patch will be proposed via gerrit.