Add ability to cancel builds that are in the waiting state

data/database.py

@@ -473,6 +473,7 @@ class BUILD_PHASE(object):
   PULLING = 'pulling'
   BUILDING = 'building'
   PUSHING = 'pushing'
+  WAITING = 'waiting'
   COMPLETE = 'complete'
 
 
@@ -491,7 +492,7 @@ class RepositoryBuild(BaseModel):
   access_token = ForeignKeyField(AccessToken)
   resource_key = CharField(index=True)
   job_config = TextField()
-  phase = CharField(default='waiting')
+  phase = CharField(default=BUILD_PHASE.WAITING)
   started = DateTimeField(default=datetime.now)
   display_name = CharField()
   trigger = ForeignKeyField(RepositoryBuildTrigger, null=True, index=True)