Fix for a caching traceback

When the user specified caching plugin doesn't initialize correctly, we
were falling back to a dict.  however, dicts do not have the same
update() method as the FactCache.  We use the update method when we
update a cache with a subsequent value.  So when that combination of
things happened, the code would traceback.

In devel, we made this change to fix things: https://github.com/ansible/ansible/pull/49516
but that involves several deprecations.  So we're doing this smaller
hack in 2.7 to fix the traceback without introducing those deprecations
in a stable release.

changelogs/fragments/vm_fix.yml

@@ -0,0 +1,2 @@
+bugfixes:
+- Fix traceback when updating facts and the fact cache plugin was nonfunctional

lib/ansible/vars/manager.py

@@ -622,10 +622,21 @@ class VariableManager:
         if host.name not in self._fact_cache:
             self._fact_cache[host.name] = facts
         else:
-            try:
-                self._fact_cache.update(host.name, facts)
-            except KeyError:
-                self._fact_cache[host.name] = facts
+            if isinstance(self._fact_cache, FactCache):
+                try:
+                    self._fact_cache.update(host.name, facts)
+                except KeyError:
+                    self._fact_cache[host.name] = facts
+            else:
+                # Dictionary fallback so we need to use a dictionary update.
+                try:
+                    host_cache = self._fact_cache[host.name]
+                except KeyError:
+                    host_cache = facts
+                else:
+                    host_cache.update(facts)
+
+                self._fact_cache[host.name] = host_cache
 
     def set_nonpersistent_facts(self, host, facts):
         '''