Add endpoint for /datastores/{datastore_rdn}/datastore_directory for job-exec-use

[linnbjornerud]

Currently we have one endpoint for /datastores/{datastore_rdn}. This is used by job-exec today (to find datastore-directory of a datastore), but we also want to retrieve information about the datastores from datastore-admin (admin-dashboard).

Suggested solution to add a new endpoint job executor can use (ref comment in: #70) that returns the directory. Secure it with an api-key.
/datastores/{datastore_rdn}/datastore_directory

[pawbu]

Check also if job-exec verifies BUMP_ENABLED or find out who does that

[pawbu]

BUMP_ENABLED is verified by datastore-api when creating the new BUMP job.