fix failing dump command

[besworks]

Reason for the change

Per Issue #225

File ".../rethinkdb/utils_common.py", line 311, in take_action
values[dest] = getpass.getpass("Password for admin: ")
TypeError: 'Values' object does not support item assignment

Description
The values object is an instance of optparse_parser.Values which, as the error message states, does not support item assignment. I modified the option checker to use the ensure_value method of the values object to set the password.

Checklist

• I have read and agreed to the RethinkDB Contributor License Agreement

[gabor-boros]

@besworks Thank you for the fix 🎉

This looks good to me! I'll merge this later today and cut a new release on Pypi.

[gabor-boros]

Although I could do the release now, I prefer doing it with fresh eyes. So I'll do it tomorrow morning as a first step after my coffee.

Sorry about any inconviniences.