Add 'use_openssl' attribute for when installing HAProxy from source.

Hello!

To be able to use haproxy's SSL support HAProxy needs to be compiled with `USE_OPENSSL=1` (from the README in the haproxy tarball).

This adds a default `use_openssl` attribute and changes the `make` command in `install_source.rb` to add the `USE_OPENSSL=1` flag when the `use_openssl` attribute is true.
The default for `use_openssl` is `false`.