Redis fixes
Closes https://gitlab.com/gitlab-org/gitlab-workhorse/issues/117
Closes https://gitlab.com/gitlab-org/gitlab-workhorse/issues/114
See also https://gitlab.com/gitlab-org/gitlab-workhorse/issues/118
- Don't panic on missing redis toml section #117 (closed)
- Fix parsing of sentinel url's
- Improve error messages
- Omit redigomock from /vendor due to licensing issues #114 (closed)