Convert snmp.yml to configdb

[tsvanduyn]

- Why I did it
This PR is in preparation to move from snmp.yml to configdb. This will more closely align with other commands in sonic and use configdb as the source of truth for snmp configuration.

Note: This is the first of 2 PR's to enable this. This PR will not change any functionality but will allow the snmp.yml file info to be put into the configdb.

Once this is verified as working as expected then the updates to the Jinja2 template to only use the snmp info from the configdb and the update to the start.sh to exclude the import of data from the snmp.yml will be done. (This will allow us backward compatiblity since the snmp.yml file info will be installed in the configdb at docker container startup with the conversion script.)

Eventually we will be able to remove the need for the snmp.yml file.

- How I did it
I've created a script that takes the snmp.yml variables and converts them to the configdb format.
In addition this file was added to the dockerfile.j2 so that file will be copied in the container.
As well as updating the start.sh file to automatically run the python conversion script each time the docker container is restarted.

- How to verify it

Before docker restart:

admin@str-s6000-acs-11:~$ sudo vim /etc/sonic/snmp.yml
snmp_location: Emerald City
snmp_rocommunities: [public, Bill, Travis]
snmp_rwcommunity: Joker

admin@str-s6000-acs-11:~$ redis-cli -n 4 keys "*" | grep -i snmp 
SNMP|CONTACT
FEATURE|snmp
admin@str-s6000-acs-11:~$ 

After docker restart:

admin@str-s6000-acs-11:~$ redis-cli -n 4 keys "*" | grep -i snmp 
SNMP|LOCATION
SNMP_COMMUNITY|Joker
SNMP_COMMUNITY|Travis
SNMP|CONTACT
SNMP_COMMUNITY|Bill
SNMP_COMMUNITY|public
FEATURE|snmp
admin@str-s6000-acs-11:~$ 

admin@str-s6000-acs-11:~$ cat /etc/sonic/snmp.yml
snmp_location: Emerald City
snmp_rocommunities: [public, Bill, Travis]
snmp_rwcommunity: Joker
admin@str-s6000-acs-11:~$ 

- Which release branch to backport (provide reason below if selected)

• 201811
• 201911
• 202006

- Description for the changelog

- A picture of a cute animal (not mandatory but encouraged)

[tsvanduyn]

retest this please

[tsvanduyn]

retest this please

[qiluo-msft]

As comments

[lguohan]

retest vs please

[lguohan]

retest vsimage please

[jleveque]

Retest vsimage please