Adds a new module ironic_python_agent.inspector and new entry point
for extensions, which will allow vendor-specific inspection.
Inspection is run on service start up just before the lookup.
Due to this early start, and due to the fact we don't even know
MAC address of nodes on inspection (to say nothing about IP addresses),
exception handling is a bit different from other agent features:
we try hard not to error out until we send at least something to inspector.
Change-Id: I00932463d41819fd0a050782e2c88eddf6fc08c6
