add special case read-only support for isolated public/private keys

We recently made AWX_ISOLATED_PRIVATE_KEY and AWX_ISOLATED_PUBLIC_KEY
read-only so that they're not inadvertently modified and/or deleted
(which would cause isolated task execution to break).  Tower's notion of
a read-only setting, though, is really more like "hard-coded in a .py
file".  What we really need is support for settings that are not
user configurable/changeable, but that we still want to display to users
from the DB.

In leiu of a complicated change to `awx.conf.settings`, this change
provides special support to these two settings so they behave in the
manner we expect.

see: https://github.com/ansible/ansible-tower/issues/7375

awx/conf/settings.py

@@ -293,7 +293,12 @@ class SettingsWrapper(UserSettingsHolder):
         field = self.registry.get_setting_field(name)
         if value is empty:
             setting = None
-            if not field.read_only:
+            if not field.read_only or name in (
+                # these two values are read-only - however - we *do* want
+                # to fetch their value from the database
+                'AWX_ISOLATED_PRIVATE_KEY',
+                'AWX_ISOLATED_PUBLIC_KEY',
+            ):
                 setting = Setting.objects.filter(key=name, user__isnull=True).order_by('pk').first()
             if setting:
                 if getattr(field, 'encrypted', False):