Add instance-filter argument

Adds --instance-filter command line argument that takes an expression to pass to the instances.list method.

A bit of framework cleanup in the process. Noting our python:3.7-slim was no longer being updated.

Note: old DB in https://console.cloud.google.com/sql/instances/sql-6e3f03c1/overview?project=uis-microservices has label for testing.

Closes: #4 (closed)