Add env var to override introspection bind address

[jacksontj]

2b08772 (inadvertently?) changed the
bind address from :61678 to localhost:61678 which is backwards
incompatible and for those actually scraping the metrics via prometheus
makes the metrics endpoint entirely unusable (since prometheus isn't
local to each node in the cluster).

This path simply adds an env var to override the bind address.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[jacksontj]

I have added another commit which changes the default back because the introspection endpoint is largely useless when bound to localhost.

[mogren]

@jacksontj Hi, the prometheus metrics port has not changed, it was just moved to another file: ipamd/metrics.go. You can still query :61678/metrics on the aws-node to get the metrics.

PR #433 moved the introspection endpoint that returns data in json format and that is used for debugging the CNI's internal state to another port and bound it to localhost: ipamd/introspect.go. The debug script was updated to handle this.

Are there some additional metrics that you are missing that can only be found through the introspection endpoints?

[jacksontj]

@mogren OIC, it changed the / response code for the metrics endpoint as well (used to be 200, now 404 -- this is what we were using for our liveliness check). I was seeing failures connecting initially but those seem to be all due to #502 .

The bind interface for this introspection endpoint was still changed without mention in the release notes. I'll remove the patch to change the default back, but I still think it would be good to add the env variable override -- but it isn't a huge blocker for my specific use-case

[jacksontj]

I also just added the new env var to README (apparently I missed that in the previous commit).

[tiffanyfay]

Looks good to me, thanks!