Add option to limit directories in git repo on the server side

[asafc]

how it works now

At the moment OPAL server tracks a git repo for sourcing policy code (rego and static data) to its clients. The entire repo is exposed by default to the clients, and the clients decide what directories that want to subscribe to.

potential improvement

The opal server should potentially accept a config option to only track some directories on the source repo (i,e: have different roots) and only tracks them.