improve #137 so we properly replace feature URLs used for creating child containers
fixes #137 to use the new ${version:foo} to refer to the version of a 'foo' value which comes from org.fusesource.fabric.version.properties in the profile; so we can easily centralise the versions of things used in various places in profiles for features URLs or bundles; so there's only 1 place we store the version for perfectus / karaf /camel etc so its easier to do consistent version upgrades on a profile baseis