WIP: Support GitLab tracing configuration

Closes #4104 (closed)

Approach

Provide a global setting gitlab_tracing_configuration which allows an administrator to configure a distributed tracing "connection string", for example:

gitlab_tracing_configuration 'opentracing://jaeger'

This setting will control multiple configurations, including:

1. The GITLAB_TRACING environment variable in GitLab-Workhorse
2. The GITLAB_TRACING environment variable in Sidekiq
3. The GITLAB_TRACING environment variable in Rails
4. The GITLAB_TRACING environment variable in Gitaly
   1. Note that Gitaly will automatically propagate the GITLAB_TRACING environment variable to Gitaly-Ruby (see labkit!10 (merged))
5. The gitlab_tracing value in config.yml in GitLab-Shell. (see gitlab-shell!277 (merged) for details as to why we don't use the GITLAB_TRACING environment variable in GitLab-Shell)