Add scopes-exclude config option for compute services

[dotsdl]

Currently, it is possible to specify which Scopes a compute service should draw Tasks from via the scopes configuration key. However, in some cases it is complementary to also be able to specify Scopes that should be excluded. This can happen when one gives a non-specific Scope in scopes, but would like to leave out selection of Tasks from a specific Scope that is a subset of that non-specific Scope.

We should add a scopes-exclude configuration option that applies after Scopes given in scopes as a filter on matches.