WIP: enable EC2 Spot usage in AWS

Change-Id: I9868d014991d78e7b2421439403ae1371b33524c

doc/source/aws.rst

@@ -66,6 +66,7 @@ Selecting the ``aws`` driver adds the following options to the
                  cloud-image: debian9
                  instance-type: t3.large
                  key-name: zuul
+                 use-spot: True
                  tags:
                    key1: value1
                    key2: value2
@@ -717,3 +718,10 @@ Selecting the ``aws`` driver adds the following options to the
 .. _`VM Import/Export service role`: https://docs.aws.amazon.com/vm-import/latest/userguide/vmie_prereqs.html#vmimport-role
 .. _`instance quotas`: https://us-west-1.console.aws.amazon.com/servicequotas/home/services/ec2/quotas
 .. _`AWS RegisterImage API documentation`: https://docs.aws.amazon.com/AWSEC2/latest/APIReference/API_RegisterImage.html
+
+           .. attr:: use-spot
+              :type: bool
+              :default: False
+
+              When set to True, nodepool will try to launch an EC2 Spot
+              instance, instead of an On-Demand instance.

nodepool/driver/aws/adapter.py

@@ -834,6 +834,16 @@ class AwsAdapter(statemachine.Adapter):
                     del mapping['Ebs']['Encrypted']
                 args['BlockDeviceMappings'] = [mapping]
 
+        # enable EC2 Spot
+        if label.use_spot:
+            args['InstanceMarketOptions'] = {
+                'MarketType': 'spot',
+                'SpotOptions': {
+                    'SpotInstanceType': 'one-time',
+                    'InstanceInterruptionBehavior': 'terminate'
+                }
+            }
+
         with self.rate_limiter(log.debug, "Created instance"):
             log.debug(f"Creating VM {hostname}")
             instances = self.ec2.create_instances(**args)

nodepool/driver/aws/config.py

@@ -177,6 +177,7 @@ class AwsLabel(ConfigValue):
         self.tags = label.get('tags', {})
         self.dynamic_tags = label.get('dynamic-tags', {})
         self.host_key_checking = self.pool.host_key_checking
+        self.use_spot = bool(label.get('use-spot', False))
 
     @staticmethod
     def getSchema():
@@ -198,6 +199,7 @@ class AwsLabel(ConfigValue):
             },
             'tags': dict,
             'dynamic-tags': dict,
+            'use-spot': bool
         }