Check for ISC DHCP plugin availability before querying for leases#487

Snuffy2 merged 3 commits intotravisghansen:maintravisghansen/hass-opnsense:mainfrom

Snuffy2:Support-removal-of-ISC-DHCPSnuffy2/hass-opnsense:Support-removal-of-ISC-DHCPCopy head branch name to clipboard