kea: fix creating runtime directory in service files

kea: fix creating runtime directory in service files

Create runtime and state directories using RuntimeDirectory and
StateDirectory in systemd service files, rather than creating them via
ExecStartPre. This is a standard approach for systemd[1]. When the
server starts, systemd will automatically create these directories.

Also, add more configuration files to CONFFILES.

[1] https://www.freedesktop.org/software/systemd/man/latest/systemd.exec.html#RuntimeDirectory=

Signed-off-by: Yi Zhao <yi.zhao@windriver.com>
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>