Allow drush plugins to replace Context classes with other classes.

I am struggling to get Hosting Docker working smoothly.

I encountered a problem with the new Composer Install feature when running on "remote" servers, see #2952997: Run composer install on remote server. for that issue.

So stock aegir only runs commands either locally or via SSH. the Provision_Context_server::shell_exec() function is hard coded that way.

I need to override Provision_Context_server::shell_exec() for docker, to run it via docker exec instead.

The only way we can do this is by adding an "alter" hook to the context loading.

Branch incoming.