(PUP-2520) implements 'manual' environment directories

Hi,

Please find here an implementation of the 'manual' type of directory environments, along with a puppet face to list the environements and flush them.

I tried to cover the whole feature with as much tests I can, but I didn't provide any acceptance tests.

It's been a long time since I contributed some code to Puppet, so please bear with me and do not hesitate to comment the following patches!

Thanks!
Brice