dhcp: T3237: add constraint to static-mapping mac-address node
dhcp: T3237: add constraint to static-mapping mac-address node

The format of the CLI specified MAC address was not validated as only addresses
with a colon as seperator are supported. Constraint has been added.

vyos/vyos-1x,vyos/vyos-1x,vyos/vyos-1x,vyos/vyos-1x