Add assign-via-gearman to config validator

Also fix fake.yaml to correctly validate.

Change-Id: I32d1980d0bd8d511d47114f45f550b43f6f58676

nodepool/cmd/config_validator.py

@@ -95,6 +95,7 @@ class ConfigValidator:
             'name': str,
             'hostname': str,
             'subnode-hostname': str,
+            'assign-via-gearman': bool,
             'jenkins': {
                 'url': str,
                 'user': str,

tools/fake.yaml

@@ -42,7 +42,6 @@ providers:
         name-filter: 'Fake'
         setup: prepare_node_devstack.sh
       - name: multi-fake
-        subnodes: 2
         base-image: 'Fake Precise'
         min-ram: 8192
         name-filter: 'Fake'