Ability to set severity of a check

#### Feature Description

Currently the check failure severity (`warning`/`failing`) is determined depending on type of check.

* Script checks switch to `warning` if the exit code is `1`
* HTTP checks switch to `warning` for `429` status code

I don't believe any other ones show this specific differentiation.

#### Use Case(s)

My use case is I'm passing the Consul alerts via [consul-alerts](https://github.com/AcalephStorage/consul-alerts) service to [VictorOps](https://victorops.com/) almost all of them appear as "Critical" when they are `failing`. But for some I'd like to lower their severity to a warning(and "Moderate" in VO) always.

For example, lets say I have an HTTP `/healthcheck` that can fail, but I might not care that much it does for this specific service and I'd like any status code other than `200` to be treated as a `warning`. For that reason I'd like a `severity` setting(or something like that) in a check definition that I could set to `warning`.

No file or test is named. Start by tracing the check definition and the existing severity rules for script exit code 1 and HTTP status 429; done means a check can explicitly set its severity to warning, including the HTTP health-check use case.

Written by the indexing model from the issue text.