Make MAC address regex check case insensitive

The regex check for the container MAC address presence in the
dnsmasq leases file is case sensitive. Dnsmasq outputs uppercase
addresses in the leases file.

As MAC addresses are generally considered case insensitive, it
makes sense to relax the regular expression check to allow for this.
Merge pull request #283 from bkkrw/master

Make MAC address regex check case insensitive