Support generic filtering by prefix in `FlagsBase`.

Currently, some overrides of FlagsBase::load() can take a prefix which is used to filter env vars read from the environment. Sometimes it is undesirable to read the environment in FlagsBase and hence the env vars are passed by the user, see e.g. MESOS-3475. In such cases it would be convenient if FlagsBase supports filtering for provided environments as well.