Give bin/storage some replica-aware options

Summary:
Fixes T10758.

• Adds a "--host" flag. If you specify this, we read your cluster config. This lets you dump from a replica.
• Adds a "--for-replica" flag to storage dump. This makes mysqldump include a CHANGE MASTER ... statement in the output, which is useful when setting up a replica for the first time.

Test Plan:

• Dumped master and replica cluster databases.
• Dumped non-cluster databases.
• Ran various other commands (help, status, etc).

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10758

Differential Revision: https://secure.phabricator.com/D15714