Update Netbox._get_snmp_config to revised logic

Next in our queue is to actually these methods, but for now,
`_get_snmp_config()` was an ugly and unnecessary copy of
`get_preferred_snmp_management_profile`.  This rewrites it to defer to
the latter, while updating the callers to the new "require_write" logic.

python/nav/models/manage.py

@@ -311,7 +311,7 @@ def read_only(self):
             category=DeprecationWarning,
             stacklevel=2,
         )
-        return self._get_snmp_config('community', writeable=False)
+        return self._get_snmp_config('community')
 
     @property
     def read_write(self):
@@ -321,7 +321,7 @@ def read_write(self):
             category=DeprecationWarning,
             stacklevel=2,
         )
-        return self._get_snmp_config('community', writeable=True)
+        return self._get_snmp_config('community', require_write=True)
 
     @property
     def snmp_version(self):
@@ -338,25 +338,16 @@ def snmp_version(self):
                 return 2
             return int(value)
 
-    def _get_snmp_config(self, variable='community', writeable=None):
+    def _get_snmp_config(self, variable='community', require_write=False):
         """Returns SNMP profile configuration variables, preferring the profile
         with the highest available SNMP version.
         """
         # TODO: This method can be removed when the SNMP properties above are removed
-        query = Q(protocol=ManagementProfile.PROTOCOL_SNMP)
-        if writeable:
-            query = query & Q(configuration__write=True)
-        elif writeable is not None:
-            query = query & (
-                Q(configuration__write=False) | ~Q(configuration__has_key='write')
-            )
-        profiles = sorted(
-            self.profiles.filter(query),
-            key=lambda p: str(p.configuration.get('version') or 0),
-            reverse=True,
+        profile = self.get_preferred_snmp_management_profile(
+            require_write=require_write
         )
-        if profiles:
-            return profiles[0].configuration.get(variable)
+        if profile:
+            return profile.configuration.get(variable)
 
     def get_preferred_snmp_management_profile(
         self, require_write=False