UrfArbitrator: allow the arbitrator to continue if /dev/rfkill is unavailable.

Signed-off-by: Mathieu Trudel-Lapierre <mathieu.trudel-lapierre@canonical.com>
Merge pull request #2 from cyphermox/without-rfkill

UrfArbitrator: allow the arbitrator to continue if /dev/rfkill is unavailable