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/node/defaults/main.yml

@@ -0,0 +1,3 @@
+---
+# Name of the master group
+group_name_master: master