Specify DB URL on command-line for schema_diff.py

The exising schema_diff code had the DB URL format hard-coded which made it
difficult to work with since different environments with have different
user/password configurations.

A more flexible way to handle this, is to have the DB URL specified directly
on the command-line.

Change-Id: I313bb616e439072e007c886a7f131efc6a61f6df
Merge "Specify DB URL on command-line for schema_diff.py"