uniter: skip unknown rel. endpoints
Ignore relations with unknown (not implemented by
the charm) when handling upgrades.
The uniter is now requesting all relation events
on upgrade, so they won't be missed.
This also fixes LP bug #1101140 (uniter does not
skip unimplemented relations).
R=fwereade, TheMue
CC=
https://codereview.appspot.com/7385049