tests_py3: decode get_file_contents() result

When running tests in python3 we get bytes object instead of
bytestring from get_file_contents() and when passing it to
run_command() we later fail on concatenation in shell_quote().

https://pagure.io/freeipa/issue/7131

Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>

ipatests/pytest_plugins/integration/tasks.py

@@ -228,7 +228,7 @@ def restore_files(host):
 def restore_hostname(host):
     backupname = os.path.join(host.config.test_dir, 'backup_hostname')
     try:
-        hostname = host.get_file_contents(backupname)
+        hostname = host.get_file_contents(backupname, encoding='utf-8')
     except IOError:
         logger.debug('No hostname backed up on %s', host.hostname)
     else: