Background

• BFE can be configured with Passive Health Check (i.e. Outlier Detection) and use responses from real requests to determine whether an backend instance is healthy.

Description
Add Active Health Check support:

• Actively and periodically send health checking requests to backend.
• Use responses from health checking requests to determine whether an backend instance is he

Currently the release script uses invoke (inv), but the way to make it work is manually installing it's dependencies (pyyaml and semver).

We should make this process straight-forward (probably add a requirements file might be enough, but if you consider it is if needed, add docs too).

currently "option forwardfor" cant't be unset, so there is no way to enable it in the default section and disable it in a particular frontend

Consul previously did not supports metadata, but now it does. We use tags instead but can now revert to using metadata. Need to update the client to use meta instead of tags. Here is our documentation ticket for this: SteeltoeOSS/Documentation#19

Add a response timeout value in the timeout message.