Add authentication support to the apt proxy plugin

Add username and password settings to the APT Proxy plugin so that
users who are behind an authenticating proxy can still use bootstrap-vz
without having to jump through hoops. If either the username or password
are not set, then no authentication is used.

bootstrapvz/plugins/apt_proxy/README.rst

@@ -14,6 +14,12 @@ Settings
     *``required``*
 -  ``port``: The port (integer) of the proxy server.
     *``required``*
+-  ``username``: The username for authentication against the proxy server.
+   This is ignored if ``password`` is not also set.
+   *``optional``*
+-  ``password``: The password for authentication against the proxy server.
+   This is ignored if ``username`` is not also set.
+   *``optional``*
 -  ``persistent``: Whether the proxy configuration file should remain on
    the machine or not.
    Valid values: true, false

bootstrapvz/plugins/apt_proxy/manifest-schema.yml

@@ -10,8 +10,10 @@ properties:
         type: object
         properties:
           address: {type: string}
+          password: {type: string}
           port: {type: integer}
           persistent: {type: boolean}
+          username: {type: string}
         required:
         - address
         - port

bootstrapvz/plugins/apt_proxy/tasks.py

@@ -34,11 +34,21 @@ class SetAptProxy(Task):
 	@classmethod
 	def run(cls, info):
 		proxy_path = os.path.join(info.root, 'etc/apt/apt.conf.d/02proxy')
+		proxy_username = info.manifest.plugins['apt_proxy'].get('username')
+		proxy_password = info.manifest.plugins['apt_proxy'].get('password')
 		proxy_address = info.manifest.plugins['apt_proxy']['address']
 		proxy_port = info.manifest.plugins['apt_proxy']['port']
+
+		if None not in (proxy_username, proxy_password):
+			proxy_auth = '{username}:{password}@'.format(
+				username=proxy_username, password=proxy_password)
+		else:
+			proxy_auth = ''
+
 		with open(proxy_path, 'w') as proxy_file:
-			proxy_file.write('Acquire::http {{ Proxy "http://{address}:{port}"; }};\n'
-			                 .format(address=proxy_address, port=proxy_port))
+			proxy_file.write(
+				'Acquire::http {{ Proxy "http://{auth}{address}:{port}"; }};\n'
+				.format(auth=proxy_auth, address=proxy_address, port=proxy_port))
 
 
 class RemoveAptProxy(Task):