Improve ways to manage manifests for when temporarily switching branches

Right now it's difficult to quickly test a branch or a completely new repository, that might be on a different commit than the HEAD of the various repos.

For example, when testing a community branch, they are often a bit outdated and sometimes require temporarily disabling certain plugins, such as the enterprise ones.
Currently one needs to manually remove the directories (or rather move them somewhere else), manually check out the remote branches to test and reverse everything once done.
That's not only error prone, but also difficult to reproduce.

If the project tool would support disabling specific plugins (e.g. by moving them to a `disabled` directory), disabling everything _except_ `graylog2-server` and the given plugin, while retaining the locally cloned repos and creating a local manifest out of this, the workflow would be much more streamlined.