Allow all SSL handlers to have a custom SSL certificate supplied

This patch introduces HandlerSSLCert as a new option for all SSL-enabled handlers (except Meterpreter, which are handled in a separate PR). 

Custom SSL certificates have been tested with the following handlers:
 * cmd/unix/reverse_bash_telnet_ssl
 * cmd/unix/reverse_openssl
 * cmd/unix/reverse_perl_ssl
 * cmd/unix/reverse_php_ssl
 * cmd/unix/reverse_python_ssl
 * cmd/unix/reverse_ruby_ssl
 * cmd/unix/reverse_ssl_double_telnet
 * nodejs/shell_reverse_tcp_ssl
 * python/shell_reverse_tcp_ssl
 * ruby/shell_reverse_tcp_ssl