Use basename when setting PS4

The PS4 mangling that devtest.sh does is very useful in showing
exactly where an issue with a devtest run is going wrong. However,
for users that have a TRIPLEO_ROOT set, this turns the PS4 prompt
into a mass of 70 lines, which harms debugging. Use basename to
only show the script itself, rather than the full path.

Change-Id: Ie55e714d50622ac38f36f67a26af5b138c1d2f48
Merge "Use basename when setting PS4"