refs #7264 - check for java, unless installing for capsule

This commit moves the java check from a check to a hook, so that
we can more easily differentiate the behavior for a capsule vs
server install.  In addition, if the installer is being run for
the capsule, skip the java check, since the capsule does not require
it.
I may be interpreting this wrong, but isn't this saying if we are deploying a capsule without a Pulp node do a version check of Java? What happens if I am deploying just a TFTP Capsule?