Fix scale attribute to accept 0 as a value

Signed-off-by: Jonathan Cremin <jonathan@crem.in>

compose/service.py

@@ -177,7 +177,7 @@ class Service(object):
         network_mode=None,
         networks=None,
         secrets=None,
-        scale=None,
+        scale=1,
         pid_mode=None,
         default_platform=None,
         **options
@@ -192,7 +192,7 @@ class Service(object):
         self.pid_mode = pid_mode or PidMode(None)
         self.networks = networks or {}
         self.secrets = secrets or []
-        self.scale_num = scale or 1
+        self.scale_num = scale
         self.default_platform = default_platform
         self.options = options