tests: Disable slow and stress tests by default

Currently developers must use this crazy long command line when
running the tests:

    NOSE_SKIP_STRESS_TESTS=1 NOSE_SKIP_SLOW_TESTS=1 make check

If you don't add the environment variables, you are wasting your time
waiting for slow and stress tests, or worse, stress tests may fail
because your system is overloaded.

This patch disables slow and stress tests by default, so the common way
to run the tests is just:

    make check

To enable slow and stress tests, you can use new shorter environment
variables:

    NOSE_STRESS_TESTS=1 NOSE_SLOW_TESTS=1 make check

The new environment variables are also documented now.

Change-Id: Id671a54261e8ef6595277b64b2e4222931ddccd6
Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/32523
Reviewed-by: Francesco Romani <fromani@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>