app: added StatusBuilding in the UnitPriority method.

related to #553.
app: added tests for StatusBuilding.
api: using building instead pending.

related to #553.
collector: using building instead pending for tests.

related to #553.
juju: using building instead pending.
provision: removed StatusPending.
docker: making IsReachable public.

It should fix the lint.

related to #553.