* Add ability to setup multiple tags for a task.
The database supports an unlimited number of tags per tasks. It would be
nice to support that feature from user side too.
* The tags feature is a bit useless, if there is no way to filter by them.
This patch adds this ability to `rally task list` command.
Change-Id: I959fa904a38f193f77f25b53ba3ee86760fff91c