Werk #12931: mk_docker: Fix crash in mk_docker agent for Python2

Component	Checks & agents
Title	mk_docker: Fix crash in mk_docker agent for Python2
Date	Sep 7, 2021
Level	Trivial Change
Class	Bug Fix
Compatibility	Compatible - no manual interaction needed
Checkmk versions & editions	2.1.0b1 Checkmk Raw (CRE), Checkmk Enterprise (CEE), Checkmk MSP (CME) 2.0.0p10 Checkmk Raw (CRE), Checkmk Enterprise (CEE), Checkmk MSP (CME)

Docker agent plugin for Python2, mk_docker_2.py crashed when fetching df data. This resulted with an UNKNOWN state in docker_node_info check.

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