Introduce get_python_exec_prefix function.

get_python_exec_prefix returns the path to the direcotry where python
executables are installed, that is /usr/bin on Fedora and /usr/local/bin
everywhere else.

It is used to properly locate OpenStack executables.

Fixes: bug #1068386

Change-Id: I228498ebe2762568d00757d065e37377ee2c8fb3
Merge "Introduce get_python_exec_prefix function."