MEDIUM: ssl: allow to change the OpenSSL security level from global section

MEDIUM: ssl: allow to change the OpenSSL security level from global section

The new "ssl-security-level" option allows one to change the OpenSSL
security level without having to change the openssl.cnf global file of
your distribution. This directives applies on every SSL_CTX context.

People sometimes change their security level directly in the ciphers
directive, however there are some cases when the security level change
is not applied in the right order (for example when applying a DH
param).

Before this patch, it was to possible to trick by using a specific
openssl.cnf file and start haproxy this way:

    OPENSSL_CONF=./openssl.cnf ./haproxy -f bug-2468.cfg

Values for the security level can be found there:

https://www.openssl.org/docs/man1.1.1/man3/SSL_CTX_set_security_level.html

This was discussed in github issue #2468.