This change allows the client HTTP and the server HTTP, Serf and
RPC health check names within Consul to be configurable with the
defaults as previous. The configuration can be done via either a
config file or using CLI flags.
Closes#3988
* Allow server TLS configuration to be reloaded via SIGHUP
* dynamic tls reloading for nomad agents
* code cleanup and refactoring
* ensure keyloader is initialized, add comments
* allow downgrading from TLS
* initalize keyloader if necessary
* integration test for tls reload
* fix up test to assert success on reloaded TLS configuration
* failure in loading a new TLS config should remain at current
Reload only the config if agent is already using TLS
* reload agent configuration before specific server/client
lock keyloader before loading/caching a new certificate
* introduce a get-or-set method for keyloader
* fixups from code review
* fix up linting errors
* fixups from code review
* add lock for config updates; improve copy of tls config
* GetCertificate only reloads certificates dynamically for the server
* config updates/copies should be on agent
* improve http integration test
* simplify agent reloading storing a local copy of config
* reuse the same keyloader when reloading
* Test that server and client get reloaded but keep keyloader
* Keyloader exposes GetClientCertificate as well for outgoing connections
* Fix spelling
* correct changelog style
Since I was already fixing consul's tls handling in #2645 I decided to
update consul/api and pre-emptively fix our tls handling against the
newest consul/api behavior. consul/api's handling of http.Transports has
improved but would have broken how we handled tls (again).
This would have made for a nasty surprise the next time we updated
consul/api.
