Reimplementation of sentries using `juju run`

This is a complete overhaul of amulet sentries. The existing sentry API is preserved, but the 'subordinate charm' approach is replaced with calls to `juju run`.

In 74210d8 I added the new implementation alongside the old, adding functional tests to ensure that the new implementation returned the same results as the old. In 36ee2ae the old implementation is completely removed.

Along with the unit and functional tests, I used the new implementation to run existing amulet tests from the charm store. Specifically, I tested with meteor, haproxy, and memcached. All amulet tests completed successfully (and passed).