engine: Fixed a spelling mistake in AuditLogAgingThreashold config var

Fixed a spelling mistake in 'AuditLogAgingThreashold' config variable,
Changed to 'AuditLogAgingThreshold'.

This patch also contains a generic new SQL function (fn_db_rename_config_key)
which renames a conf key while preserving the current conf value.

Change-Id: I14becc0f57404a86ca18917e4b6e2d00054a44b1
Bug-Url: https://bugzilla.redhat.com/743032
Signed-off-by: Asaf Shakarchi <asaf@redhat.com>