use Slurm as the default job backend + deprecate support for GC3Pie as job backend

[gkaf89]

• The GC3Pie back-end for job management that has been the default so far is no longer actively supported. Use the Slurm back-end as default instead.
• Add deprecation message when the GC3Pie class is instantiated.

fixes #4565

[gkaf89]

There is a test that still fails after the latest commit:

test.framework.parallelbuild.ParallelBuildTest.test_build_easyconfigs_in_parallel_gc3pie

I traced the issue in the deprecation message which was added in the GC3Pie constructor. Adding a deprecation message to the log during testing raises an Exception. The test succeeds if the exception is caught. Under normal operation, logging the deprecation message does not raise any exception.

Is there any special handling of deprecation message logging in the tests? How should I handle exceptions caused by deprecation messages in tests?

[boegel]

Is there any special handling of deprecation message logging in the tests? How should I handle exceptions caused by deprecation messages in tests?

@gkaf89 Just call self.allow_deprecated_behaviour() in the test, there's a bunch of examples of this already in the test suite