spec: make libattr-devel a Build dependency for everything Lustre

Change-Id: I42d43a4698ec4cd02cfadd3dcdbc6c87ffef409d
Signed-off-by: Thomas Leibovici <thomas.leibovici@cea.fr>
rbh-config: backup_db use option --single-transaction of mysqldump.

By default, mysqldump locks the whole tables to backup them,
which blocks any robinhood activity during the backup.
Using --single-transaction option with InnoDB avoids this wide locking
whereas the backup is still consistent.

Change-Id: I3f0bb92335dc4fe531ec74c1295a7ba8d8722529