Improve the way we switch between development and GA

JBDS-3190 has shown that there are too many changes to perform when willing to create a GA candidate, and it's almost certain that we'll forever forget to change one or some of them when switching between GA and development stream.
We need to improve that.

Changes are necessary in:

• features/com.jboss.devstudio.core/feature/p2.inf
• site/associate.properties
• results/pom.xml

As an alternative, I suggest that the final site be ALWAYS added to the referenced site, even if it's empty. This has no cost for build nor user, and this would simplify a few things here and there.
Also, instead of a p2.inf, we could think a a "startup" extension that would add reference to development site in case qualifier for the feature doesn't contain GA.
The property to the "current site" (GA or development) could be factorized in JBDS parent pom. so that both results/pom.xml and site/pom.xml could use it (instead of associateSites.properties).

CC nickboldt manderse@redhat.com