[#2032] Update build system to buildbot changes.

Problem description
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Buildbot configuration was changed to reduce the customization on buildbot part and user more script from the repo.



Changes description
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Add a new test_ci paver task which will be used by buildbot for running tests.

Update merge_init and merge_commit to get arguments  from environment.


How to try and test the changes
===============================

reviewers @alibotean 

All developer commands should work as in previous version... only buildbot infrastructure was changed... but end user commands are the same 

test_remote and pqm is the same.

there are now group and gk test_remote targets.

'test_remote all' was not rename into 'test_remote gk-review' to signal that it will run the test required to pass a review

'test_remote pqm' was renamed to 'test_remote gk-merge' to signal that it will do the merge... but this was guarded by our 'paver pqm' command so you should not feel this change.

This is on top of #2030 but 2030 can not be merged now since without having this landed pqm does not work.

This is to let you know about the changes.

Thanks!