Suppress `unknown parameter: ...` warning logs when running `rain deploy` with `--ignore-unknown-params`

[railsharipov]

Warning logs can get noisy when running deploys for large CFN stacks. I have multiple CFN stack templates that share the same Rain parameter file, some parameters are shared between stacks and some are not.

It seems reasonable to suppress these logs completely when user explicitly provides --ignore-unknown-params flag.

As an alternative, it would be nice to have a flag to suppress warnings.