Validation passed to Expectations inconsistently

[scotthellman]

Most expectations take a validation upon construction, but http expectations take a validation through their validate method, and ssh expectations take validations at both times. For consistency and to ensure that we don't report two different expectations as being the same (eg to pagerduty), we need to have our expectations all using the same pattern.

Passing in a validation at construction seems like the best option, for the above-mentioned pagerduty reason.