Make systemd vs rht detection more robust for centos

Centos 6.x and below use an old RHT style of configuring hostname.
CentOS 7.x and better use systemd.  Instead of depending on the
distribution string which seems to have changed over the course of 6.x
we need to explicitly check the version.

Fixes #8997

system/hostname

@@ -367,12 +367,20 @@ class RedHatWorkstationHostname(Hostname):
 class CentOSHostname(Hostname):
     platform = 'Linux'
     distribution = 'Centos'
-    strategy_class = RedHatStrategy
+    distribution_version = _get_distribution_version()
+    if distribution_version and LooseVersion(distribution_version) >= LooseVersion("7"):
+        strategy_class = FedoraStrategy
+    else:
+        strategy_class = RedHatStrategy
 
 class CentOSLinuxHostname(Hostname):
     platform = 'Linux'
     distribution = 'Centos linux'
-    strategy_class = FedoraStrategy
+    distribution_version = _get_distribution_version()
+    if distribution_version and LooseVersion(distribution_version) >= LooseVersion("7"):
+        strategy_class = FedoraStrategy
+    else:
+        strategy_class = RedHatStrategy
 
 class ScientificHostname(Hostname):
     platform = 'Linux'