Fix post install scriptlet

We need to add the tomcat user to katello-shared for katello-all and
headpin-all, because that are the packages with a dependency on
tomcat.

Otherwise the scriptlet are run before tomcat is even installed, which
causes cpinit to fail in katello-configure.
Merge pull request #1367 from iNecas/katello-shared-fix

Fix post install scriptlet