Add version option to docker_daemon plugin

As requested on #147, there is now an option to specify the Docker
version to be installed when using the `docker_daemon` plugin. The
version string is validated against a pattern extracted from the
Docker's CHANGELOG. If the version is not present, it will just download
the latest available.

The download method was also changed from `urllib` to `wget`, so we can
see its progress if needed.

This closes #147.

bootstrapvz/plugins/docker_daemon/manifest-schema.yml

@@ -3,6 +3,15 @@ $schema: http://json-schema.org/draft-04/schema#
 title: Install Docker plugin manifest
 type: object
 properties:
+  plugins:
+    type: object
+    properties:
+      docker_daemon:
+        type: object
+        properties:
+          version:
+            pattern: '^\d\.\d{1,2}\.\d$'
+            type: string
   system:
     type: object
     properties:

bootstrapvz/plugins/docker_daemon/tasks.py

@@ -25,13 +25,18 @@ class AddDockerDeps(Task):
 class AddDockerBinary(Task):
 	description = 'Add docker binary'
 	phase = phases.system_modification
-	DOCKER_URL = 'https://get.docker.io/builds/Linux/x86_64/docker-latest'
 
 	@classmethod
 	def run(cls, info):
-		import urllib
+		from bootstrapvz.common.tools import log_check_call
+		docker_version = info.manifest.plugins['docker_daemon'].get('version', False)
+		docker_url = 'https://get.docker.io/builds/Linux/x86_64/docker-'
+		if docker_version:
+			docker_url += docker_version
+		else:
+			docker_url += 'latest'
 		bin_docker = os.path.join(info.root, 'usr/bin/docker')
-		urllib.urlretrieve(cls.DOCKER_URL, bin_docker)
+		log_check_call(['wget', '-O', bin_docker, docker_url])
 		os.chmod(bin_docker, 0755)