Enable $pybasedir value in pxe.py

We should not refer $pybasedir directly in our python code, because
it is loaded by paths.py, we should use paths.basedir_def("") to load
this $pybasedir value as a base path.

And change some 'nova' words to 'ironic in paths.py

Change-Id: I4c6bcb103d4d1eee8e7bf78f46c815630666cfe9
Closes-Bug: #1267351
Merge "Enable $pybasedir value in pxe.py"