configverify: T6198: add common helper for PKI certificate validation

The next evolutional step after adding get_config_dict(..., with_pki=True) is
to add a common verification function for the recurring task of validating SSL
certificate existance in e.g. EAPoL, OpenConnect, SSTP or HTTPS.

(cherry picked from commit 3b758d870449e92fece9e29c791b950b332e6e65)