BUILD: Add a few more targets for intg tests
    
    Running "make intgcheck" has been proven to be a bit painful (mainly
    when the developer is just writing down a single test case), as it
    cleans up the build directory and fireis a new build before, finally,
    run the tests.
    
    In order to make it a little less painful, let's break the whole
    operation into 3 new targets:
        intgcheck-{prepare,run,clean}.
    
    As expected, "make intgcheck" calls these 3 new operations in the same
    order they were presented, not changing then the current behavior.
    
    Each operation will trigger the previous one in case there is no
    "$$prefix" directory created and the directory is _only_ created in the
    very first operation (intghcheck-prepare).
    
    A note must be done about how to run a simple test file or a simple test
    from a test file when running "make intgcheck-run". The option always
    been here but only makes sense now that we have the intgcheck split in a
    few useful steps. See the examples below (and for more detailed
    information, check the py.test documentation):
     #Run a single file
     make intgcheck-run INTGCHECK_PYTEST_ARGS="-k test_netgroup.py"
     #Run a single test from a single file
     make intgcheck-run INTGCHECK_PYTEST_ARGS="-k test_add_empty_netgroup"
    
    Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>