cmdlib: Stop forking in LUClusterQuery

While debugging another issue we realized that LUClusterQuery forks.
This turned out to be the “platform.architecture” function from the
Python library. It uses the “file” command to determine the architecture
of the Python binary.

This patch adds two new functions to the “runtime” module to get this
information once per process instead of doing it every single time
LUClusterQuery is used. Forking is a no-go in a multi-threaded
environment anyway.

A future change will also have to change the terminology in “gnt-cluster
info”: it reports the binary architecture simply as “architecture”, when
it's actually the binaries' architecture. Kernel and userland can be
different.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Bernardo Dal Seno <bdalseno@google.com>