Improve schema upgrade workflow for unprivileged users

Summary:
In a basically reasonable configuration where you connect
with a non-privileged user from the web workflow, upgrade_schema.php
won't have enough privileges. Allow the user to override the normal
auth with -u and -p.

Test Plan:
Tried to do a schema upgrade with an underprivileged user,
got a useful error message instead of garbage.

Reviewed By: Girish
Reviewers: Girish, davidrecordon, jungejason, tuomaspelkonen, aran
CC: aran, epriestley, Girish
Differential Revision: 191