Directly link providers and request handlers

Rather than relying on inspection and loading classes from specific
filenames, ask each driver's Provider implementation to supply the
NodeRequestHandler instance for requests.

This is the first in a series of changes to remove all similar
indirection from the drivers in order to make them easier to
understand.

The test driver is moved out of the fixtures subdir to be a
"regular" driver because it now needs relative imports (provider
code needs to import the handler code).

Change-Id: Iad60d8bfe102d43b9fb1a1ff3ede13e4fa753572

nodepool/driver/fake/provider.py

@@ -23,6 +23,7 @@ import shade
 
 from nodepool import exceptions
 from nodepool.driver.openstack.provider import OpenStackProvider
+from nodepool.driver.fake.handler import FakeNodeRequestHandler
 
 
 class Dummy(object):
@@ -303,3 +304,6 @@ class FakeProvider(OpenStackProvider):
             self.createServer_fails -= 1
             raise Exception("Expected createServer exception")
         return super(FakeProvider, self).createServer(*args, **kwargs)
+
+    def getRequestHandler(self, poolworker, request):
+        return FakeNodeRequestHandler(poolworker, request)