FR: allow for a 2nd password param for seamless changes

For a more seamless password changing procedure it would be nice to be able to temporarily set two passwords for a server, so that you could do:

• configure 2nd password on maxscale side
• change password on server side
• remove old password and make new one the standard one on maxscale side

This way maxscale config change would not need to be perfectly synced with the password change on the server side, maxscale could continue to try the old password first, and the new one on failure of the old one, or vice versa, or always try the one that succeeded on the previous attempt so that only one login failure due to wrong password would happen.

Especially when having multiple maxscale instances there might be a significant time window for the complete password change rollout, and this way we could guarantee that it would still work without any outages.