Add a way to pass SSL key and certificate to enable Secure FTP (SFTP).

vsftpd supports the following via its conf file:

ssl_enable=YES
allow_anon_ssl=NO
force_local_data_ssl=YES
force_local_logins_ssl=YES
ssl_tlsv1=YES
ssl_sslv2=NO
ssl_sslv3=NO
require_ssl_reuse=NO
ssl_ciphers=HIGH
rsa_cert_file=/etc/vsftpd.pem
rsa_private_key_file=/etc/vsftpd.pem
pasv_max_port=65535
pasv_min_port=64000