[show][config] Add new snmp commands (sonic-net#1347)

Added new SNMP show and config commands using ConfigDB as well as unittests.
show commands: 
show runningconfiguration snmp 
show runningconfiguration snmp contact [--json] 
show runningconfiguration snmp location [--json] 
show runningconfiguration snmp community [--json] 
show runningconfiguration snmp user [--json] 

config commands:
sudo config snmp community add/del/replace
sudo config snmp contact add/del/modify
sudo config snmp location add/del/modify
sudo config snmp user add/del

Author: tsvanduyn