Logic proposal for --force flag to delete the validators during client bulk delete.

:construction: 

This is my proposal for adding --force logic to client bulk delete. Once we agree on the logic I'll go ahead and add the specs, etc... and complete the PR. 

Proposed Logic: 

If some validators match the regexp user provided to client bulk delete chef says `Following clients are validators and will not be deleted. You must specify --force to delete the validator clients.` and asks if the user wants to delete the matching non-validator clients.

When `--force` option is specified chef first asks if it's OK to delete the validators after that asks again to delete the non-validator clients.

Alternatively we can make it so that chef doesn't ask twice when `--force` option is specified. I thought it is better to point out the validator clients separately even when we are deleting them so that user is fully aware of what they are doing.

Thoughts @opscode/client-eng & @curiositycasualty ?