Do not use "del os.environ" as the variable might not exist

The use of del os.environ assumes that the environment variable exists.
If the variable does not exist, this call will result in a traceback.
The solution is to use os.environ.pop(VARIABLE, None) instead.

This is the ansible-freeipa fix for https://pagure.io/freeipa/issue/9446
(Nightly test failure for replica installation with --setup-ca)

plugins/module_utils/ansible_freeipa_module.py

@@ -226,7 +226,7 @@ def temp_kdestroy(ccache_dir, ccache_name):
     """Destroy temporary ticket and remove temporary ccache."""
     if ccache_name is not None:
         run([paths.KDESTROY, '-c', ccache_name], raiseonerr=False)
-        del os.environ['KRB5CCNAME']
+        os.environ.pop('KRB5CCNAME', None)
     if ccache_dir is not None:
         shutil.rmtree(ccache_dir, ignore_errors=True)