Add keyring operations to the HTTP API

[slackpad]

The keyring ops available here should be available via an HTTP endpoint. Any automation around these now requires scripting command line ops or using undocumented client RPC methods (which we'd like to turn down).

#1408

[ryanuber]

Just a quick note - I believe we did this because the HTTP interface was commonly exposed, and originally had no HTTPS support. It would be nice to warn/log or deny plain HTTP requests for keyring operations.

[slackpad]

This is now https://www.consul.io/api/operator/keyring.html. We didn't end up doing the HTTP deny part since HTTPS support is available, and that seems like too much of a special case.