add support for persisting to object store

READY FOR REVIEW - now includes HA, CLI.

basis is to refactor file-based support to use a common PersistenceObjectStore interface backed by either filesystem or jclouds blobstore (running w softlayer only so far, but using jclouds).

to run integration tests against SL, set up a named location such as the following:

    brooklyn.location.named.softlayer-objectstore-amsterdam-1=jclouds:swift:https://ams01.objectstorage.softlayer.net/auth/v1.0
    brooklyn.location.named.softlayer-objectstore-amsterdam-1.identity=IBMOS1234:yourname
    brooklyn.location.named.softlayer-objectstore-amsterdam-1.credential=....

on reflection it does feel like the right place.  i've added support for setting `PERSISTENCE_BACKUP_REQUIRED` which is unsupported for clouds, and the (new) impl for JcloudsPersistObjStore only does anything on CLEAN when it blows it away