state/apiserver: keep backups in env storage

Whenever the backup API is called keep the backup tarball in
environment storage in addition to returning it to the client. Among
other things, this will be used to support rollback when upgrades
fail.
state/apiserver: close backup files once returned
Merge pull request #278 from mjs/backups-in-env-storage

state/apiserver: keep backups in env storage

Whenever the backup API is called keep the backup tarball in environment storage in addition to returning it to the client. Among other things, this will be used to support rollback when upgrades fail.

There is more work to be done to add APIs to support management of these backups. This will be done at a later date by the Moonstone team.