[BuildStream] Deprecating docker plugin in bst-external

[Jim MacArthur <jim macarthur codethink co uk>]

I would like to mark the docker source plugin in bst-external [1] as 
deprecated.

At the moment, it is deprecated, but there's no indication that it is 
deprecated; it's been moved to the bst-plugins-container repository. I 
worked with the old version for about a day; luckily, there haven't been 
any changes since it was moved.

Given the recent discussion about deprecating plugins [2] I'd like to 
suggest that we make a standard warning token called "deprecated" for 
plugins, so we at least have the chance to silence deprecation warnings 
in future if we decide to go that way. I have no idea how we actually 
record that standard in BuildStream yet.

Other than that, if I just add a warning with that token and appropriate 
text in bst-external's docker plugin, would anyone object?

[1] 
https://gitlab.com/BuildStream/bst-external/blob/master/bst_external/sources/docker.py

[2] 
https://mail.gnome.org/archives/buildstream-list/2018-November/msg00098.html