Add es package

Looking for some feedback on this approach to talking to ES.
I understand that this code is using precedent set by mgo.MgoTestPackage,
but I'm not sure it will work well for us. Consider that we will almost certainly
want to have some tests in internal/v4 that test how the API works along
with elastic search, which means we will need both mongo and elastic search
to run at the same time. With this approach, that's not possible, because
we can't have tests running both within MgoTestPackage and ElasticSearchTestPackage
at the same time.

Perhaps I could suggest another approach, one that we were considering for juju-core too: require that the user start an elastic search instance before running the tests. Then all packages can use the same elastic search engine, and we won't be hit by 6s startup times (and potentially multiple concurrent running JVMs).

To do this, we'd need two things (at least):

 - an environment variable that points to the elastic search engine to use (perhaps ELASTICSEARCH=host:port; or ELASTICSEARCH=none to disable the elastic search tests)
- the ability to tell the code to use a given prefix on all indexes and schemas

Then we'd have an ElasticSearchSuite that generated a random prefix, probably including the date too, and used that for the index/schema prefix. It would delete all its indexes and schemas at the end, and possibly garbage-collect old ones so we don't accumulate endless garbage indexes from interrupted tests.

This means that most of the code in this file would be unnecessary. What do you think?