Add redis auth support

[munnerz]

This PR adds support for redis authentication through the REDIS_PASSWORD environment variable.

For a production-ready environment this is a requirement for us!

[guoyoujin]

@munnerz That's a good idea, that is what I need,Thank you

[stale]

This issue has been automatically marked as stale because it has not had any activity for the last 60 days. It will be closed if no further activity occurs during the next 7 days. Thank you for your contributions.

[solidnerd]

Hey @munnerz,
I know this pr is a bit older. I would like to merge this into master. Could you rebase your pr?

[mpv945]

rake aborted!
URI::InvalidURIError: bad URI(is not URI?): "redis://{{REDIS_PASSWORD}}redis:6379/0"
/home/git/gitlab/lib/gitlab/redis/wrapper.rb:148:in redis_store_options' /home/git/gitlab/lib/gitlab/redis/wrapper.rb:120:in params'
/home/git/gitlab/lib/gitlab/redis/wrapper.rb:20:in params' /home/git/gitlab/lib/gitlab/redis/wrapper.rb:111:in redis'
/home/git/gitlab/lib/gitlab/redis/wrapper.rb:31:in `block in pool'