kea: add kea-uci package with basic init script

kea: add kea-uci package with basic init script

This init script allows to start the Kea Control Agent, the DHCPv4
server, the DHCPv6 server, and the DHCP-DDNS server. It expects the
config files to be where the packages install them.

As this is a single init script that can start 4 different binaries that
are each in their own package, these files cannot be included in any of
these other package, so create a dedicated package for it.

Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>