Initial snmp agent integration
Basic agent to manager communication working
Add terminate callback to exometer report plugins.

Also snmp is only started if the export is configured to be enabled.
Move main exometer eunit test to CT.

Also adds skeleton for snmp test suite.
Removed old unused tests
Add SNMP startup configuration tests
Working agent-manager communication tests

Exometer won't start the SNMP application anymore implicitely. The user
is required to properly configure and start SNMP in its node when
trying to use SNMP export in exometer.