[New "nginx_ssl_manage_certs" variable]

Added a new "nginx_ssl_manage_certs" variable that makes the
process of copying certificates to the remote machine optional,
while still allowing the user to enable SSL.

This means that users can run a 'vault' based SSL key copying
mechanism, while still using this role.
Merge pull request #1 from unboxed/master

[New "nginx_ssl_manage_certs" variable]