Added run_gaiatest_desktopb2g.py

ex:
`./run_gaiatest_desktopb2g.py --testvars <TESTVARS>`
or
`./run_gaiatest_desktopb2g.py --testvars <TESTVARS> --tests gaia/tests/python/gaia-ui-tests/gaiatest/tests/functional/browser/test_browser_lan.py`
or
`./run_gaiatest_desktopb2g.py --testvars <TESTVARS> --type=b2g-smoketest --tests gaia/tests/python/gaia-ui-tests/gaiatest/tests/functional/browser/manifest.ini`
or
`./run_gaiatest_desktopb2g.py --testvars <TESTVARS>  --desktopb2g_dir <PATH_TO_DESKTOP_B2G> --tests gaia/tests/python/gaia-ui-tests/gaiatest/tests/functional/browser/test_browser_lan.py`