ringaccount: use relative paths in configuration

By default for new accounts, use relative paths
for certificates, keys and archive.
Absolute paths are still supported.
This make it easier to move ring installs between
computers.

Change-Id: I5a34c8174225446e6b5d6cb17c05f147cdff0b5b
Reviewed-by: Guillaume Roguez <guillaume.roguez@savoirfairelinux.com>