deployment: make the keyholder key path configurable

This is the correct fix for bug T75520 - contrary to what was done in
I08c24578596506a1a8baedb7f4a42c2c78be295a we do not change the logic of
the class, we just allow to change the key location via hiera, and leave
the default value to be the one we use in production.

Change-Id: Iba742c94aa3df7497fbff52a856d7ba16cf22cc7
Signed-off-by: Giuseppe Lavagetto <glavagetto@wikimedia.org>