support redis timeout option for cache and data conns

test plan:

- add a `timeout: 1` option to redis.yml
- confirm that rails cache and redis operations complete as normal
- to test timeout, you can set up a dummy tcp server that listens but
  never accepts, add it to the redis config, and verify that the (first)
  redis operation times out as expected. after the first, it'll be
  blacklisted for 5m as usual.

Change-Id: I2494b4b319afffcf1ea6ab87eb1c3d7d3baec58b
Reviewed-on: https://gerrit.instructure.com/39895
Reviewed-by: Jacob Fugal <jacob@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Brian Palmer <brianp@instructure.com>
QA-Review: Brian Palmer <brianp@instructure.com>