Make grant loader local key domain prefix configurable

# What? 

The domain prefix used in localKeys in grant loader needs to be configurable. It is currently hard coded for the JhuUpdater which is the default updater.

# Why?

So that if other institutions want to use the JhuUpdater default update logic, they can set the local key domain prefix to something appropriate for them.

# How?


# Acceptance Criteria 


# Related Issues

