[config]: Add hostname command to config CLI

[samaity]

Signed-off-by: Sangita Maity sangitamaity0211@gmail.com

- What I did
added new command change-hostname to config CLI so that the user can change the hostname on a SONiC device without traffic being impacted.

- How I did it
update "hostname" key to the "DEVICE_METADATA|localhost" field in config DB using sonic-cfggen and restart the hostname-config service.

- How to verify it

changed the hostname from csw06 to csw06-sonic

admin@csw06:~$ sudo config change-hostname csw06-sonic
Running command: service hostname-config restart
Please note loaded setting will be lost after system reboot. To preserve setting, run config save.


admin@csw06:~$ sudo config save -y
Running command: /usr/local/bin/sonic-cfggen -d --print-data > /etc/sonic/config_db.json


admin@csw06:~$ exit
logout
Connection to 172.x.x.x closed.


➜  ~ ssh admin@172.x.x.x
admin@172.x.x.x's password:
Linux csw06-sonic 4.9.0-9-amd64 #1 SMP Debian 4.9.168-1+deb9u3 (2015-12-19) x86_64
Unauthorized access and/or use are prohibited.
All access and/or use are subject to monitoring.


Help:    http://azure.github.io/SONiC/


Last login: Tue Sep 24 18:11:57 2019 from 172.x.x.x
admin@csw06-sonic:~$    -> **hostname got updated successfully**

[jleveque]

Please simplify the command syntax from config change-hostname <new_hostname> to config hostname <new_hostname>

[jleveque]

Retest this please

[jleveque]

Retest this please

[jleveque]

Retest this please

[jleveque]

Retest this please

[jleveque]

Retest this please