proxy support for apt::ppa

If the node is behind a proxy, the apt class has two parameters to describe the proxy configuration (`proxy_host` and `proxy_port`).

Due to a bug in add-apt-repository, the settings is ignored, and you must define the environment variable `https_proxy`. See [launchpad bug](https://bugs.launchpad.net/ubuntu/+source/software-properties/+bug/516032) for more details.

The code I propose modifies the environment of the exec resource if the apt class is defined with a `proxy_host` value different from the default (false).