salt.returners.influxdb_return: add no-op save_minions() func for API compatibility

doc/ref/returners/all/salt.returners.influxdb_return.rst

@@ -4,3 +4,4 @@ salt.returners.influxdb_return
 
 .. automodule:: salt.returners.influxdb_return
     :members:
+    :exclude-members: save_minions

salt/returners/influxdb_return.py

@@ -149,6 +149,13 @@ def save_load(jid, load):
         log.critical('Failed to store load with InfluxDB returner: {0}'.format(ex))
 
 
+def save_minions(jid, minions):  # pylint: disable=unused-argument
+    '''
+    Included for API consistency
+    '''
+    pass
+
+
 def get_load(jid):
     '''
     Return the load data that marks a specified jid