Add python2-sqlalchemy-collectd

This allows monitoring of SQLAlchemy connection pool
usage.

Change-Id: I2bcaa44c184bb24e5caeeb8d204dda627de59c91

docker/collectd/Dockerfile.j2

@@ -48,7 +48,8 @@ LABEL maintainer="{{ maintainer }}" name="{{ image_name }}" build-date="{{ build
       'collectd-write_http',
       'collectd-write_kafka',
       'collectd-write_prometheus',
-      'python-collectd-gnocchi'
+      'python-collectd-gnocchi',
+      'python2-sqlalchemy-collectd'
   ] %}
 
   {% if base_arch in ['x86_64', 'ppc64le'] %}