Riak Linux support - improvements

[Valentin Aitken <va...@cloudsoftcorp.com>]

Hi all,

I am working on improving the support for the Distributed Database Riak.
I would be very glad if you review my pull request and consider merging 
because we are planning more improvements and I do not want to merge too 
much things.
https://github.com/apache/incubator-brooklyn/pull/531

Basically now, It is installing the latest version of riak community 
edition from packagecloud.io
Besides installing from packagecloud it also support fallback mechanism 
for installing from the default location specified in the BrooklynNode 
interface
If an explicit location is specified then it is preferred over the 
packagecloud.

Kind regards,
Valentin!