Fix Vault certificates configuration

Summary:
salt["file.file_exists"]("/some/dir") returns False, so check a file will do.

Also, HCL defines options using the key = value syntax.

This is a follow-up for 7700b529.

Test Plan: salt-call --local state.sls roles/vault/vault test=True should be no-op on Complector.

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D2640