Improved feedback for domU install on XS

Change-Id: I5b9d07493eb334169fb2643047a014f56ee265fa
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
Improve millisecond logging

Always display 3 digits for milliseconds (appends 0s)
Based on I3f1461839258be0723e2d3616ec225a830d13029

Change-Id: I48b3f3781b4d34ed6a5fb9e4e78cee919afda6c1
Merge "Improved feedback for domU install on XS"
Merge "Improve millisecond logging"
Merge "Introduce get_python_exec_prefix function."