BUG/MINOR: ssl: shut the ca-file errors emitted during httpclient init

With an OpenSSL library which use the wrong OPENSSLDIR, HAProxy tries to
load the OPENSSLDIR/certs/ into @system-ca, but emits a warning when it
can't.

This patch fixes the issue by allowing to shut the error when the SSL
configuration for the httpclient is not explicit.

Must be backported in 2.6.
3 files changed