Replace hardcoded 'master' group name with 'group_name_master' variable (#337)

For improved flexibility and maintainability.

* Update tasks in node role to use 'group_name_master' variable instead
  of hardcoded 'master' group name
* Update tasks in master role to use 'group_name_master' variable instead
  of hardcoded 'master' group name
* Update tasks in post role to use 'group_name_master' variable instead of
  hardcoded 'master' group name

Signed-off-by: Christian Berendt <berendt@23technologies.cloud>

roles/k3s_server_post/defaults/main.yml

@@ -1,3 +1,6 @@
 ---
 # Timeout to wait for MetalLB services to come up
 metal_lb_available_timeout: 120s
+
+# Name of the master group
+group_name_master: master