Reduce verbosity of tests

SlapOS test is too verbose, see http://community.slapos.org/test_result_module/20181211-426F13FE/6 as an example. The main goal of this work is to make this easier to read.

Other improvements:

• introduce subprocess32 on python2 to use new subprocess features on python2
• honor SLAPOS_TEST_DEBUG environment variable to output logs and also enable debugging utilities ( unittest's Ctrl+C handler )