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

src/libostree/ostree-sysroot-private.h

@@ -121,4 +121,10 @@ gboolean _ostree_sysroot_piecemeal_cleanup (OstreeSysroot *sysroot,
                                             GCancellable *cancellable,
                                             GError **error);
 
+gboolean _ostree_sysroot_write_deployments_internal (OstreeSysroot     *self,
+                                                     GPtrArray         *new_deployments,
+                                                     OstreeSysrootCleanupFlags cleanup_flags,
+                                                     GCancellable      *cancellable,
+                                                     GError           **error);
+
 G_END_DECLS