module docker_image - default tag latest

Fix constant rebuild of image if no tag specified.

library/cloud/docker_image

@@ -164,7 +164,7 @@ class DockerImageManager:
         for i in images:
             # Docker-py version >= 0.3 (Docker API >= 1.8)
             if 'RepoTags' in i:
-                repotag = '%s:%s' % (getattr(self, 'name', ''), getattr(self, 'tag', 'latest'))
+                repotag = ':'.join([self.name, self.tag])
                 if not self.name or repotag in i['RepoTags']:
                     filtered_images.append(i)
             # Docker-py version < 0.3 (Docker API < 1.8)
@@ -188,7 +188,7 @@ def main():
         argument_spec = dict(
             path            = dict(required=False, default=None),
             name            = dict(required=True),
-            tag             = dict(required=False, default=""),
+            tag             = dict(required=False, default="latest"),
             nocache         = dict(default=False, type='bool'),
             state           = dict(default='present', choices=['absent', 'present', 'build']),
             docker_url      = dict(default='unix://var/run/docker.sock'),