snmp_agent: generate table OID based on instance as a key in case index is not specfi...

snmp_agent: generate table OID based on instance as a key in case index is not specfified.

Based on the SNMP rules when table column is represented as a key
of string type, key should be used while generating OID for each row.
When index is not specified, then it's assumed that instance value is
the key for the specified table.

Change-Id: I9f3fbd9aa018bc9037bd3202e2ed107a302fe1b3
Signed-off-by: Korynkevych, RomanX <romanx.korynkevych@intel.com>