Log policy: Improve acces logging

Feature requests:

• Ability to customize access log format per service (some custom format or JSON for example)
  • We would have to wait for next OpenResty release to properly support this. THREESCALE-1234
• Ability to enable / disable those logs conditionally per service GH #856
• Should be able to ship logs to a syslog server GH #858
• Ability to configure the location of the access log file GH #650

Reference: https://github.com/nmasse-itix/apicast-logger
GitHub issue: https://github.com/3scale/apicast/issues/712