MINOR: ssl: render file-access optional on server crt loading

MINOR: ssl: render file-access optional on server crt loading

The function ssl_sock_load_srv_cert will be used at runtime for dynamic
servers. If the cert is not loaded on ckch tree, we try to access it
from the file-system.

Now this access operation is rendered optional by a new function
argument. It is only allowed at parsing time, but will be disabled for
dynamic servers at runtime.