Migrated test towards unit-style.

This test was intended to check the internal behavior of a type and thus is more of a unit test. In this spirit, we've migrated more towards this style by stubbing an internal variable to observe the type's behavior.
State upgrade step: Add EnvUUID to charm docs.

Add an upgrade step to add the environment uuid to charm docs. Update
doc struct, call sites and tests respectively.
State upgrade step: Add EnvUUID to minUnits docs.

Add an upgrade step to add the environment uuid to minUnits docs. Update doc struct, call sites and tests.
fixed conflicts
Refactor backups metadata storage to use an external session.
Use a new backups for each API request (and close when finished).
Switch backups archive storage to gridfs.
Run -> RunTransaction.
NewBackupsStorage -> NewBackupStorage.
raw -> managed.
Remove superfluous interface tests.
envFileStorage -> backupBlobStorage.
Switch back to the more compact txn Ops formatting.
Drop the error return arg from newBackups.
Adjust to state.NewBackups.
Add a TODO about getting the machine ID.
Add DBOperator.RunTransaction.
Add DBOperator.Metadata.
c.Check -> c.Assert.
Add DBOperator.TxnOp.