ComposeUserData provides no mechanism for customising apt options

We've got environs.ComposeUserData, which takes a set of additional scripts that are added to the cloudinit.Config structure after running the ConfigureBasic/ConfigureJuju functions over MachineConfig

 In the MAAS provider we need to also do an apt-get update && apt-get install bridge-utils during bootstrap. This is necessary so that we restart networking before the synchronous bootstrap phase begins. Ideally we would just set the appropriate options on cloudinit.Config directly, but instead we're currently stuck with adding runcmds.

See: https://codereview.appspot.com/77890045/