Why I did it
Part implementation of dhcp_server. HLD: sonic-net/SONiC#1282.
Add dhcpservd to dhcp_server container.
How I did it
Add installing required pkg (psutil) in Dockerfile.
Add copying required file to container in Dockerfile (kea-dhcp related and dhcpservd related)
Add critical_process and supervisor config.
Add support for generating kea config (only in dhcpservd.py) and updating lease table (in dhcpservd.py and lease_update.sh)
How to verify it
Build image with setting INCLUDE_DHCP_SERVER to y and enabled dhcp_server feature after installed image, container start as expected.
Enter container and found that all processes defined in supervisor configuration running as expected.
Kill processes defined in critical_processes, container exist.
