Allow assignments to be conditional too #72

Putting together some decent examples, in #66, made me consider how other people might approach problems.

I tend to use wget for downloading binaries, for example. But these days curl is perhaps more common.

It might be nice to support this:

let download=""

let download="/usr/bin/curl" if exists(/usr/bin/curl)
let download="/usr/bin/wget" if exists(/usr/bin/wget)

fail { message => "Can't find download program", if empty(download)}

Of course there's the obvious argument "If you want to use wget just .. install it":

package { name => "wget", state => "installed", name => "wget:package" }

shell { name => "download" , require => "wget:package" }

Still supporting conditionals in rules and includes does mean that assignments are the odd one out ..