Add regex-based response validation to Cloudprober

[manugarg]

It would be great if Cloudprober's HTTP probes could also do some simple regex-based response validation.

[manugarg]

a5ac6dd added validators framework to cloudprober. This framework allows adding validators to probes. Currently there are two validators available -- HTTP status code and data integrity. Regex based validator will be added soon.

[manugarg]

Commit cd0610a adds regex based validator. I'll soon add an example to demonstrate how this works.

[manugarg]

Added example here:
https://github.com/google/cloudprober/blob/master/examples/validators/cloudprober_validator.cfg