Cleaner fix for JBIDE-13599..

Based on snjeza's patch but witout hard dependency on dmr, with no
dependency on having properties set in pom (can run tests from the
IDE), more tests and fully functional ${foo,bar:default} syntax as use
in iin jboss.

Adds suport for ${foo}, ${foo,baz}, ${foo:default}, ${foo,baz:default}
for the site url in the discovery url mechanism.
switch from stable/juno to development/kepler as default site to use in Central (JBIDE-13767)