[fix] Wrap measurement name in double quotes in timeseries_migrate co…

…mmand

Bug:
The READ_QUERY was failing if the measurement name starts with an
integer.

openwisp_monitoring/monitoring/migrations/influxdb/influxdb_alter_structure_0006.py

@@ -12,7 +12,7 @@
 SELECT_QUERY_LIMIT = 1000
 WRITE_BATCH_SIZE = 1000
 READ_QUERY = (
-    "SELECT {fields} FROM {measurement}"
+    "SELECT {fields} FROM \"{measurement}\""
     " WHERE content_type='{content_type_key}'"
     " AND object_id='{object_id}'"
 )