Decrease admin context usage in _get_guest_config

commit 594b18a3 change the context for _get_guest_config
usage to either context given or admin context if it's not
specified. However, it has side effect on later functions
which use the context, so the scope of change the
context should be decreased and this patch fixed that bug.

Change-Id: I7bd96e2c3ee691bae51ac055f57650c14d4e10b4
Merge "Decrease admin context usage in _get_guest_config"