Fix Neutron precheck to not fail with newer Docker

With newer Docker versions `systemctl show docker` returns:

  MountFlags=shared

Instead of:

  MountFlags=1048576

This fix accepts either value as valid to ensure the check is not
erroneously failing.

Closes-Bug: #1791365

Change-Id: I2bd626466d6a0e189e0d85877b2be8f2b4bb37f4