Unify providers' state storage implementations.

Unify providers' state storage implementations.

I've been asked to unify the environs.Environ.StateInfo() implementations that
are shared just about verbatim between the EC2, OpenStack MAAS, and now the
Azure providers.  But those build on bootstrapState, loadState, and saveState
which are also all duplicated between those providers.

And so here I unify those implementations.  A provider is still free to
implement its own alternative, and it would be easy enough to extend the
signatures for LoadState and SaveState so that other providers could extend
their own versions of BootstrapState but still use the same code.

Once this is done, I can create a shared implementation of StateInfo() which
will sit in the same file, and make use of LoadState/SaveState.  Even more
code can disappear.

Looking at the code, you'll note that LoadState/SaveState no longer involve an
environment.  All they care about actually is an environs.Storage
implementation, and with the decoupling, it becomes very easy to run the tests
against the localstorage implementation.

https://code.launchpad.net/~jtv/juju-core/shared-state/+merge/172373

(do not edit description out of merge proposal)

[jtv.canonical, 2013-07-01 17:29:07]

Please take a look.

[jameinel, 2013-07-01 18:55:02]

Very nice to see this code merged together.

LGTM

[fwereade, 2013-07-02 10:02:25]

Lovely, LGTM