sysroot: Avoid double cleanup, and ensure no cleanup if specified

sysroot: Avoid double cleanup, and ensure no cleanup if specified

Since forever, we've been doing two cleanups.  In
https://github.com/ostreedev/ostree/commit/8ece4d6d51bdbe3e41ab318259276bb83e553aa0
I thought we were doing just one and wanted to go to zero (if specified),
but I actually just dropped one cleanup.

In https://github.com/projectatomic/rpm-ostree/pull/452
@jlebon pointed out the duplication.  Fix this by creating a new internal
deploy wrapper that takes cleanup flags.

(Since we already had the "piecemeal cleanup" API internally, let's
 frame it in terms of that, rather than passing down a boolean).

Closes: #500
Approved by: jlebon