Add support for inspection using ironic-inspector

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

ironic_python_agent/errors.py

@@ -321,3 +321,9 @@ class DeviceNotFound(NotFound):
 
     def __init__(self, details):
         super(DeviceNotFound, self).__init__(details)
+
+
+# This is not something we return to a user, so we don't inherit it from
+# RESTError.
+class InspectionError(Exception):
+    """Failure during inspection."""