Fix distro tree url which can cause RHEV dynamic guest boot failure

RHEV dynamic guests boot up with local images while installing, the
images url are generated by os.path.join which doesn't strip the
tailing spaces in distro tree url, and if the distro tree url is not
ending with '/', there is problem too.

Bug: 912242
Change-Id: I69c6968ba9be30b2b6d3934af59a306d25d2eeba
Merge "Fix distro tree url which can cause RHEV dynamic guest boot failure" into develop