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put back `related_predecessor_but_with_different_title_than_catalog_title` which was removed by commit 85c7fb26

/reviewed-on !80

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 )


/reviewed-on nexedi/slapos.core!73

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/reviewed-on !75

Use logger which is only enabled in debug mode.
Also don't print tips when executing this file, describe it in the
docstring instead.

Especially, don't use None which cause output on the console.

to keep tests silent.

On python2, this uses subprocess32, which backports DEVNULL (and
default setting of closing file descriptors).

subprocess32 backports the good things from python3's subprocess and is
recommended on https://docs.python.org/2/library/subprocess.html

it's now configured in tests/__init__.py

When running tests we don't want to clutter the output with logs.
Environment variable SLAPOS_TEST_DEBUG can be set to restore the
verbosity

Instead of printing when passed log=True, always log at DEBUG level,
application can decide what to do with these logs.

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This will let the connection parameters with multilines value be correctly displayed.

I also fixed the linter.

/reviewed-on !74

slapos_crm: testSlapOSCRMSkins: The order of the elements of portal_catalog are not always the same.

Probably incomplete as we need to check for activities.

/reviewed-on !71

/reviewed-on nexedi/slapos.core!70

[slapos_crm] fix failing crm skins tests after include error message in generated ticket for hosting subscription