Werk #11978: ceph_df: support for octopus (15.2.6) and nautilus (14.2.15) with JSON output

This werk will make the ceph plugin more resilient against subtle output changes in future versions of the ceph command line tool. This is achieved by changing the evaluation to JSON output. The ceph agent plugin must be redeployed in order to activate this new evalution method. When the new agent plugin is deployed the numbers that are reported by the ceph checks will change slightly, because the json output is more precise. If the JSON section cannot be generated the old output method without json will be used.

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