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.c

@@ -1591,15 +1591,11 @@ ostree_sysroot_simple_write_deployment (OstreeSysroot      *sysroot,
       added_new = TRUE;
     }
 
-  if (!ostree_sysroot_write_deployments (sysroot, new_deployments, cancellable, error))
+  if (!_ostree_sysroot_write_deployments_internal (sysroot, new_deployments,
+                                                   postclean ? OSTREE_SYSROOT_CLEANUP_ALL : 0,
+                                                   cancellable, error))
     goto out;
 
-  if (postclean)
-    {
-      if (!ostree_sysroot_cleanup (sysroot, cancellable, error))
-        goto out;
-    }
-
   ret = TRUE;
  out:
   return ret;