Add a query parameter to health check to optionally return 200 on standby nodes.

[bkrodgers]

Per https://groups.google.com/forum/?utm_medium=email&utm_source=footer#!msg/vault-tool/px3M0fg3d7g/2se9LpEg2DYJ --

It would be useful to have an option to tell the health check page to return a 200 code for standby nodes, as long as they've been unsealed. This way, they are in the load balancer and ready to take over for a failed active node as soon as Vault notices and elects a new leader. This behavior can be made optional by defining a query parameter to control it, in case people prefer the current behavior. The downside of having standby units in the LB is that some traffic will have to hop through a standby node. I think that's a worthwhile tradeoff for being able to recover from a failure faster, but others can choose the current path if they want.

I'm thinking there's no need to expose an option to return a 200 for a node that's up and sealed, as such a node really is not ready to handle any traffic. But as long as it's up and unsealed, it's ready to go.