Automatically initialise elasticsearch.

Charmstore will now automatically initialise elasticsearch when starting up.
This ensures that elasticsearch will always have the correct index and mapping
configuration required for correct operation.
Perhaps given the name, it might be worth adding:

// Note that, unlike in memory mutexes, the methods on this
// type are not safe to use concurrently.

?

Alternatively, add a mutex and make it safe (which is probably my preferred option actually)