BUG/MINOR: ssl: fix ssl-{min,max}-ver with openssl < 1.1.0

BUG/MINOR: ssl: fix ssl-{min,max}-ver with openssl < 1.1.0

In bug #676, it was reported that ssl-min-ver SSLv3 does not work in
Amazon environments with OpenSSL 1.0.2.

The reason for this is a patch of Amazon OpenSSL which sets
SSL_OP_NO_SSLv3 in SSL_CTX_new(). Which is kind of a problem with our
implementation of ssl-{min,max}-ver in old openSSL versions, because it
does not try to clear existing version flags.

This patch fixes the bug by cleaning versions flags known by HAProxy in
the SSL_CTX before applying the right ones.

Should be backported as far as 1.8.