Merge branch 'master' into ab-debian

Merge branch 'master' into ab-debian

* master: (38 commits)
  python readme update: web_log
  python readme update: smartd_log
  python readme update: nginx_log removed, postgres added
  allow the installer to check configs signatures when the BASH is older than v4
  disable /var/lib/docker/* by default
  disable by default monitoring /snap/*; disable also ram* and loop* block devices
  fix gauge gradient colors; fixes #1136
  UrlService: custom_url support for _get_data and minor fixes
  elasticsearch_plugin: "python-requests" requirement removed
  prevent cpufreq messages flood; fixes #1933
  gitignore freeipmi.plugin
  allow custom defined information on the dashboard; fixes #1936
  database size for mirrored hosts is now aligned to page size
  add backwards compatibility for http listen backlog setting
  make sure disconnected_time is updated before decrementing senders
  automatic naming for libvirtd virtual machines; fixes #1045
  prevents hosts from being removed during initialization
  added compile option NETDATA_VERIFY_LOCKS to enable the locks
  added compile option NETDATA_VERIFY_LOCKS to enable the locks
  print the lock pointer in debug mode
  ...