Cleanup mount teardown.
Cleanup mount teardown.

Rework the logic to process the mounts in reverse alphanum ordering; we do
this so that we can fail out if a unmount fails (previously the code ignored
any failures).

Additionally, remove the unecessary grep, and suppress command output unless
it's actually doing something we care about.

BUG=None
TEST=Leave a stale mount in place, invoke cbuildbot, verify it cleanses.

Change-Id: I75998e5e2351630e7dbdf079d7a373dfeb162a1a
Reviewed-on: https://gerrit.chromium.org/gerrit/20026
Tested-by: Brian Harring <fdc2d0a2e2340b623587cee5ea8dff425d1eb1f3@chromium.org>
Reviewed-by: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>
Commit-Ready: Brian Harring <fdc2d0a2e2340b623587cee5ea8dff425d1eb1f3@chromium.org>

bpsinc-native/src_third_party_chromite,coreos/chromite,zhang0137/chromite,chadversary/chromiumos.chromite,chadversary/chromiumos.chromite,zhang0137/chromite,coreos/chromite,bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite,coreos/chromite,zhang0137/chromite