add further tests for tls configuration

Allow to set the total memory of an agent in its configuration file. This
can be used in case the automatic detection doesn't work or in specific
environments when memory overcommit (using swap for example) can be
desirable.

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

better api example, add telemetry documentation

This PR allows tuning of heartbeat TTLs. An example of very aggressive
settings is as follows:

```
server {
  heartbeat_grace = "1s"
  min_heartbeat_ttl = "1s"
  max_heartbeats_per_second = 200.0
}
```

Need to pointerify it to default to true since we can't tell false from
unset if it's not a pointer.

The host UUID isn't unique in many virtualized cases and of dubious
value even when it is univerally unique. Default to a random UUID.

Went overboard before I realized there's only one test case.

This PR:
* Uses Go 1.8 executable lookup
* Stores any err message from stats init method
* Allows overriding of Cpu Compute for hosts where it can't be detected

This PR introduces a parallelism limit during garbage collection. This
is used to avoid large resource usage spikes if garbage collecting many
allocations at once.

Check the capabilities of the Vault token to ensure it is valid and also
allow targetting of a role that the token is not from.

* Added the keygen command

* Added support for gossip encryption

* Changed the URL for keyring management

* Fixed the cli

* Added some tests

* Added tests for keyring operations

* Added a test for removal of keys

* Added some docs

* Fixed some docs

* Added general options