kea: set correct permissions for /var/run/kea

kea: set correct permissions for /var/run/kea

Set the permissions of /var/run/kea to 750 to fix kea server startup
error:

ERROR [kea-dhcp4.dhcp4/445.140718820303936] DHCP4_INIT_FAIL failed to
initialize Kea server: configuration error using file
'/etc/kea/kea-dhcp4.conf': 'socket-name' is invalid: socket
path:/var/run/kea does not exist or does not have permssions = 750

This permission check was introduced by commit[1] in kea 2.6.3.

[1] https://gitlab.isc.org/isc-projects/kea/-/commit/43bba7799f6892f739b4745b35bbeacef3645ad3

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