dhcp: T3316: Adjust kea lease files' location and permissions

Move the kea lease file to a separate directory /config/dhcp that kea
process can write to so that subprocesses spawned by kea process can
operate on the lease files.

To allow kea process to write to /config/dhcp, add _kea user to
vyattacfg group. And the lease files are owned completely by _kea
user to play well with kea-lfc process.

Specifically, this is necessary for kea-lfc which is spawned by kea
process to clean up expired leases. Since kea-lfc creates additional
backup lease files, it needs write access to the lease file directory.

Additionally, change the extension of the lease file from .leases to
.csv to reflect the actual file format.