snmp_agent: Implement SNMP Agent plugin.

snmp_agent: Implement SNMP Agent plugin.

The SNMP Agent plugin is an AgentX subagent that receives and handles
queries from SNMP master agent and returns the data collected by read
plugins. The SNMP Agent plugin handles requests only for OIDs specified
in configuration file. To handle SNMP queries the plugin gets data from
collectd and translates requested values from collectd's internal
format to SNMP format. This plugin is a generic plugin and cannot work
without configuration. For more details on AgentX subagent see
<http://www.net-snmp.org/tutorial/tutorial-5/toolkit/demon/>

Change-Id: I0e82131685f2138e1af0a9283b06f85dd2028878
Signed-off-by: Korynkevych, RomanX <romanx.korynkevych@intel.com>