If you want to install the default set of controllers and the image-*
controllers, at present you have to list every single one of them.
An improvement on this is to let people specify what they want _in
addition_ to the default controllers. This commit adds an argument
`--extra-components` which appends to the (most likely, default value)
slice of `--components`.
Signed-off-by: Michael Bridgen <michael@weave.works>
75023011d3