in_calyptia_fleet: download and load fleet files.

Download fleet files to a sub-directory with the timestamp of the relevant
configuration and allow it to be loaded by changing to that directory
when fluent-bit is reloaded.

  * Recurse through fleet files and download each one.
  * Change to the fleet files directory on reload so they can be referred
    to from the main fleet configuration file.
  * Delete old fleet file directories when the configuration is deleted.

Signed-off-by: Phillip Whelan <[email protected]>