BUG/MEDIUM: ssl: initialize correctly ssl w/ default-server

BUG/MEDIUM: ssl: initialize correctly ssl w/ default-server

This bug was introduced by d817dc73 ("MEDIUM: ssl: Load client
certificates in a ckch for backend servers") in which the creation of
the SSL_CTX for a server was moved to the configuration parser when
using a "crt" keyword instead of being done in ssl_sock_prepare_srv_ctx().

The patch 0498fa40 ("BUG/MINOR: ssl: Default-server configuration ignored by
server") made it worse by setting the same SSL_CTX for every servers
using a default-server. Resulting in any SSL option on a server applied
to every server in its backend.

This patch fixes the issue by reintroducing a string which store the
path of certificate inside the server structure, and loading the
certificate in ssl_sock_prepare_srv_ctx() again.

This is a quick fix to backport, a cleaner way can be achieve by always
creating the SSL_CTX in ssl_sock_prepare_srv_ctx() and splitting
properly the ssl_sock_load_srv_cert() function.

This patch fixes issue #1488.

Must be backported as far as 2.4.