system-test: build authorized-keys file

The admin interface supports authentication via SSH pub/priv keys,
which are often stored in well-known locations.

This patch takes the user's public keys, if found, and uses them to
build an authorized-keys file for the admin interface.

Target: master
Requires-notes: no
Requires-book: no
Patch: http://rb.dcache.org/r/5754/
Acked-by: Gerd Behrmann