Creating EntityPlugin for Hardware Inventory.

This interface contains two basic implementations:

1. SnmpEntityPlugin, to parse the entPhysicalTable from the ENTITY-MIB
and populate the custom attributes using vendor specific implementations
based on hardware-inventory-adapter-config.xml.

2. ScriptEntityPlugin, to execute Groovy/BeanShell/Jython scripts to
generate the hardware entity structure for non-SNMP devices. This plugin
has not been implemented yet.