Allow servicedelegation_normalize_principals to accept simple strings

The function servicedelegation_normalize_principals acn be used to
normalize principals, but would work only on list or tuples. With
this patch a simple string can be passed as the 'principal' argument
and will also be normalized.

plugins/module_utils/ansible_freeipa_module.py

@@ -733,6 +733,8 @@ def _check_exists(module, _type, name):
 
     ipa_realm = module.ipa_get_realm()
     _principal = []
+    if not isinstance(principal, (list, tuple)):
+        principal = [principal]
     for _princ in principal:
         princ = _princ
         realm = ipa_realm