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
add comment about why snapshot/backup have no lock check

This comment is intended to reduce further confusion about why a
lock check hasn't been implemented for snapshot and backup.

Change-Id: I8d89ca7b15d6713221af2d0535f9ed338ca70612
Related-Bug: #1314741
Merge "add comment about why snapshot/backup have no lock check"
Merge "Specify DB URL on command-line for schema_diff.py"