Add --max parameter in bin/flakytests to specify max number of flaky tests to fetch
Add --max parameter in bin/flakytests to specify max number of flaky tests to fetch

Summary: So we can decide how many flaky tests we want to fetch without having to redeploy.

Test Plan:
- Ran `bin/flakytests -d 2015-05-15 -m 0` in localhost and it didn't created any row in flakyteststat.
- Ran `bin/flakytests -d 2015-05-15 -m 1` in localhost and it created one row in flakyteststat.

Reviewers: haoyi

Reviewed By: haoyi

Subscribers: changesbot, mkedia

Differential Revision: https://tails.corp.dropbox.com/D111477

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