Use sane OS-dependent defaults for SSL configuration

Current code assumes /etc/ssl exists, which may not be the case.
Attempt to guess sane default for a few OS.

BUG: 1129939
Change-Id: I0f3168f79b8f4275636581041740dfcaf25f3edd
Signed-off-by: Emmanuel Dreyfus <manu@netbsd.org>
Reviewed-on: http://review.gluster.org/8790
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>