Expose slot number from metastatic driver

The static driver now exposes the slot number in the ZK node
record.  Do the same for the metastatic driver so that Zuul can
use it.

Change-Id: I41492c53f5e21e63eddecc1ac0fe5e50c50f298b

nodepool/driver/statemachine.py

@@ -161,6 +161,7 @@ class StateMachineNodeLauncher(stats.StatsReporter):
         node.region = instance.region
         node.az = instance.az
         node.driver_data = instance.driver_data
+        node.slot = instance.slot
 
         # Optionally, if the node has updated values that we set from
         # the image attributes earlier, set those.
@@ -742,6 +743,7 @@ class Instance:
     * az: str
     * region: str
     * driver_data: any
+    * slot: int
 
     And the following are even more optional (as they are usually
     already set from the image configuration):
@@ -766,6 +768,7 @@ class Instance:
         self.region = None
         self.metadata = {}
         self.driver_data = None
+        self.slot = None
 
     def __repr__(self):
         state = []