charms/openmotion: force UTF-8 locale in hook

Workaround for:
  https://github.com/juju/juju/issues/133
charms/node-app: import data on mongodb-relation-joined

Data import takes quite a long time, and we only want to do so when
we join the relation with the database, not every time the relationship
changes.
charms/mongodb: specify source=ppa:juju/stable

The default version of mongodb in precise is too old (2.0.x) as it lacks
geoindexing and there is no mongodb for trusty.

However, we can use the mongodb version (2.4.6) provided for the juju
bootstrap node from juju-core's PPA.
charms/node-app: rejigger mongodb-relation-[joined|changed]

We only want to import data once, when the mongodb relation is joined.

However, all the configuration code for getting the address of the
mongodb server and its port lives in the 'relation-changed' hook, which
only runs *after* the 'relation-joined' hook.

We really want the node-app's config/config.js to be updated on both
'relation-joined' and 'relation-changed'. So in order to avoid repeating
ourselves, combine the scripts, add a bit of logic to detect which
hook we are called as, and then do the data import on the 'relation-joined'
hook only.
hooks/node-app: add some debug output