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.
I guess we wouldn't *need* to put all the possible values in. I understand the
reluctance though. At least we should document where it might be appropriate
to use these constants. I *presume* from inspection of the code that they're
intended for the version type argument to PutDocumentVersionWithType
but it would be nice to say so.