Add improved support for packaging CloudStack for CentOS 7

This feature splits the packaging for CentOS 6 and CentOS 7 again. With the introduction of tomcat7 and systemd to much changes to keep it in the same spec.

Move the package command to a higher level directory and added an option to specify for which platform to build (-d centos7|centos63).

Tested by creating a management server using the RPMs and configuring cloudstack to run with Xenserver (6.4.99). This pretty much checks startup and the ability to do database upgrades and find hypervisor scripts.