allow custom alternatives priority

On RHEL, if you install the package java-1.7.0-openjdk, it will set its priority to 170051. But if you use the cookbook, it sets it to 1100. That's problematic, because it's lower than java-1.5.0-gcj's priority at 1500. java-1.5.0-gcj is a dependency of Tomcat.

To counter that, I added the option of specifying a custom priority number.