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Revert "openvpn: T56: remove strict checks for tls cert-file and key-file"

Description

Revert "openvpn: T56: remove strict checks for tls cert-file and key-file"

This reverts commit c414479fdf1d5ad77170f977481fb9197c9559ae.

This commit broke the smoketests and also OpenVPN complains:
Options error: You must define certificate file (--cert) or PKCS#12 file (--pkcs12)