Fix Python3 get_python_exec_prefix on Fedora 27

On Fedora 27, the default Pytho 3 install prefix changed from /usr
to /usr/local:
https://fedoraproject.org/wiki/Changes/Making_sudo_pip_safe

Closes-Bug: #1741901
Change-Id: Id40620efdf173189df053b5d380a801092933f83
1 file changed