Allow manual /postinstall loopback mounting on debug builds.

Postinstall testing requires to mount a filesystem and relabel its
files to postinstall_file. While this task will normally be performed
by the update_engine daemon running in a domain of the same name, we
also test this workflow with sample images from /data/nativetest in
eng builds.

This hides the log messages from the 'su' context when mounting and
relabeling a filesystem onto the postinstall mountpoint.

Bug: 27272144
Bug: 26955860
TEST=m; update_engine_unittests pass Postinstall tests.

Change-Id: Id39aa1afdc11a6f59434873e68a53cbcb6ae363f