Specify networks in Compose file

There's not yet a proper way for services to join networks

Signed-off-by: Aanand Prasad <aanand.prasad@gmail.com>

compose/network.py

@@ -0,0 +1,57 @@
+from __future__ import absolute_import
+from __future__ import unicode_literals
+
+import logging
+
+from docker.errors import NotFound
+
+from .config import ConfigurationError
+
+
+log = logging.getLogger(__name__)
+
+
+class Network(object):
+    def __init__(self, client, project, name, driver=None, driver_opts=None):
+        self.client = client
+        self.project = project
+        self.name = name
+        self.driver = driver
+        self.driver_opts = driver_opts
+
+    def ensure(self):
+        try:
+            data = self.inspect()
+            if self.driver and data['Driver'] != self.driver:
+                raise ConfigurationError(
+                    'Network {} needs to be recreated - driver has changed'
+                    .format(self.full_name))
+            if data['Options'] != (self.driver_opts or {}):
+                raise ConfigurationError(
+                    'Network {} needs to be recreated - options have changed'
+                    .format(self.full_name))
+        except NotFound:
+            driver_name = 'the default driver'
+            if self.driver:
+                driver_name = 'driver "{}"'.format(self.driver)
+
+            log.info(
+                'Creating network "{}" with {}'
+                .format(self.full_name, driver_name)
+            )
+
+            self.client.create_network(
+                self.full_name, self.driver, self.driver_opts
+            )
+
+    def remove(self):
+        # TODO: don't remove external networks
+        log.info("Removing network {}".format(self.full_name))
+        self.client.remove_network(self.full_name)
+
+    def inspect(self):
+        return self.client.inspect_network(self.full_name)
+
+    @property
+    def full_name(self):
+        return '{0}_{1}'.format(self.project, self.name)