Built-in checks or check scripts

Related: https://github.com/hashicorp/consul/issues/1538

#### Feature Description

I'm seeing a lot of `check_mem` and `check_cpu` scripts being used in the [documentation](https://www.consul.io/docs/agent/checks.html#check-definition), and I would like to see these get shipped with Consul. Maybe as either a repository of bash scripts or built right into Consul itself. Some examples include:

* Check Memory which will warn or fail when a node has exceeded some used memory limit
* Check CPU which will warn or fail on anomalous CPU behaviour (high iowait or usage)
* Check Disk which will warn or fail when one or more disks are running out of space
* Check Service which will warn or fail when a Systemd process is experiencing issues

This could also expand further like [Telegraf](https://github.com/influxdata/telegraf) where you can provide scripts for things like checking Nginx, Redis, Aerospike, etc. I can envision many of these, built right into Consul similar to how TTL and Script checks are done.

#### Use Case(s)

Most nodes will end up having the same set of tests, these might as well ship with Consul.