(SIMP-10204) Ensure testing with CentOS 8.4 (#51)

- update the EL8 nodes in the acceptance tests
  to run with generic/centos8 to make sure they using
  CentOS 8.4

SIMP-10204 #comment update useradd
SIMP-10252 #close

spec/acceptance/nodesets/default.yml

@@ -11,7 +11,7 @@ HOSTS:
       - server
       - default
     platform:   el-8-x86_64
-    box:        centos/8
+    box:        generic/centos8
     hypervisor: <%= hypervisor %>
     yum_repos:
       chef-current: