Testnode environment variables and log URL

- use environment variables to pass SLAPOS_TEST_LOG_DIRECTORY and SLAPOS_TEST_SHARED_PART_LIST, as discussed on nxdtest!2
- Fix #20200514-218C705 - [testnode] frontend for log access

See merge request !1304
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passed ERP5.CodingStyleTest-Master

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passed ERP5.PerformanceTest-Master

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failed ERP5.UnitTest-Master

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passed SlapOS.Eggs.UnitTest-Master.Python2

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passed SlapOS.Eggs.UnitTest-Master.Python3

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