Re-enable plugins ability to send local_settings

In the past the plugin interface was renamed but the context was
not updated meaning that subordinates could no longer send plugin
data. This change fixes the relation name the context uses and
updates the Newton template to render these settings (earlier
templates already do this)

Closes-Bug: 1578824

Change-Id: I57e4e526019bc91bdf1df77c60da33ae108e0b49

hooks/horizon_contexts.py

@@ -272,7 +272,7 @@ class LocalSettingsContext(OSContextGenerator):
 
         relations = []
 
-        for rid in relation_ids("plugin"):
+        for rid in relation_ids("dashboard-plugin"):
             try:
                 unit = related_units(rid)[0]
             except IndexError:

templates/newton/local_settings.py

@@ -973,3 +973,4 @@ ALLOWED_PRIVATE_SUBNET_CIDR = {'ipv4': [], 'ipv6': []}
 #USER_TABLE_EXTRA_INFO = {
 #   'phone_num': _('Phone Number'),
 #}
+{{ settings|join('\n\n') }}