Allow the setting of the community string stored in snmp traps in the SNMP adapter

The SNMP adapter includes the hardcoded "public" community with each generated trap.

The community string in traps is optional and in most cases of no use to a monitoring manager. A manager can always restrict the trap it receives based on the originating host/port combination. Those are set in snmp-adaptor.sar/managers.xml.

However, it would be nice to be able to set the community string in traps (or choose to not set it at all) per receiving manager.

This means that the configuration file for monitoring manager would have to be extended, e.g.:

<manager>
<address>localhost</address>
<port>1162</port>
<local-address></local-address>
<local-port></local-port>
<community>custom community string</community> <=== HERE
<version>1</version>
</manager>

For backwards compatibility, absense of the <community> element could be interpreted as "public", while an empty element could mean "no community string"