Configure dhcp_relay container hostname from config DB

[pyadvichuk]

- What I did
Implemented as a result of discussion in #2806.

• Configure dhcp_relay docker hostname with the value received from the config DB in order to fix dhcp_relay CT which failed because the "sonic" hostname was always received.
• fixed sed introduced in #2773 as inline operation can't be applied to /etc/hosts inside docker container

admin@bfn-switch-l1:~$ docker exec -it dhcp_relay bash
root@sonic-switch-l1:/# cat /etc/hosts
127.0.0.1       localhost
127.0.0.1 sonic-switch-l1
root@sonic-switch-l1:/# sed -i '/\ssonic-switch-l1$/d' /etc/hosts
sed: cannot rename /etc/sedJugNFO: Device or resource busy

- How I did it
Configure dhcp_relay docker hostname in the same way as this is done in hostname-config.sh

- How to verify it
1) Run dhcp_relay CT and verify it will pass
2)
2.1) manually change 'hostname' in config_db.json
2.2) run sudo config reload -y
2.3) check hostname inside dhcp_relay container

- Description for the changelog
Set proper hostname for dhcp_relay container on startup

@qiluo-msft please review and merge this.

[qiluo-msft]

&& [](start = 31, length = 2)

Better use ; #Closed

[pyadvichuk]

done

[jleveque]

If this is truly an issue, we should find a global way to set the hostname in all Docker containers. I'm not a fan of treating certain containers differently regarding their hostname.