Add improved support for packaging CloudStack for CentOS 7
Convert the usage server service to systemd
Fix python reference
Merge branch 'feature/centos7-rpm' of https://github.com/apache/cloudstack
CLOUDSTACK-6893: fix enum ValueOf issue which causes systemvm fail to start

(cherry picked from commit 63ff5a7cbc3341809884e47796476d47ace03961)
(cherry picked from commit d0e0edca111feb71e7cd8267d9c28820d85b12f9)