Install kernel headers before kernel devel

kernel-headers is a dependency of kernel-devel. Sometimes, when installing kernel-devel, a different version of kernel-headers is pulled in, which is wrong.
This commit ensures both kernel-headers and kernel-devel have consistent kernel version of the running cluster

Author: hanwen-cluster

cli/src/pcluster/resources/imagebuilder/update_and_reboot.yaml

@@ -277,6 +277,7 @@ phases:
                     yum install -y ./${!PACKAGE}.rpm
                   fi
                 else
+                  yum -y install kernel-headers-$(uname -r)
                   yum -y install kernel-devel-$(uname -r)
                 fi