MINOR: ssl: Add 'show ssl providers' cli command and providers list in -vv option

MINOR: ssl: Add 'show ssl providers' cli command and providers list in -vv option

Starting from OpenSSLv3, providers are at the core of cryptography
functions. Depending on the provider used, the way the SSL
functionalities work could change. This new 'show ssl providers' CLI
command allows to show what providers were loaded by the SSL library.
This is required because the provider configuration is exclusively done
in the OpenSSL configuration file (/usr/local/ssl/openssl.cnf for
instance).
A new line is also added to the 'haproxy -vv' output containing the same
information.