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.

Backport of I0f3168f79b8f4275636581041740dfcaf25f3edd

BUG: 1138897
Change-Id: I972c26236cbf070f15c3846add059bd33d60216d
Signed-off-by: Emmanuel Dreyfus <manu@netbsd.org>
Reviewed-on: http://review.gluster.org/8861
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>