Add 'enabled' flag to tracing config

[benbjohnson]

Currently, the trace log is enabled when tracing.path is set. However, it can be useful to selectively choose which node has tracing enabled.

This PR adds a tracing.enabled field which defaults to true. If it is true and the tracing.path is set then the trace log is enabled. This PR was tested manually.

tracing:
  enabled: ${FLY_REGION == 'syd'}
  path:    "/var/log/litefs/trace.log"