Merge lp://staging/~mvo/ubuntu/vivid/ubuntu-core-config/lp1460152-workaround into lp://staging/ubuntu/wily/ubuntu-core-config

ok, I'll manually stage this by:
- installing 15.04 stable
- changing channels.ini to edge
- snappy update'ing
- remount 'other' and insert this
- reboot
- profit

The code looks good, very clever of putting it here instead of the snappy code base itself :-)

I guess
+After=etc-writable.mount
just means after we can write to partitions (the systemd unit names still confuse me a bit).

Yeah, there is one more gotach here, there needs to be a "Before=ubuntu-snappy.frameworks-pre.target" to ensure the apparmor re-generation is all done before systemd starts our frameworks/app services.

I added the Before= in r30 but did not test it (sorry! time is short here).

@sergio The testplan looks great, you may need to change both channels.ini to edge, thats the only thing I can thing of. Many many thanks for testing this and lets hope we can land a fix inside apparmor sooon(ish) as I feel that will be much cleaner :)

@After> yeah, it means that the writable mounts should all be in place. And yes, its confusing! I'm not even sure this is needed but I added it to be on the safe side.

manual steps worked.

From the bug:
"FYI, the hash approach is slow for the normal case since we always have to perform an sum. Furthermore it doesn't take into account #include'd files that might also change (eg, apparmor is updated and has a different base abstraction). For the workaround, I guess it is ok since the slowdown will only be for a couple of profiles but I would have rather seen us unconditionally invalidating the cache when switching from a to b or vice versa."