Avoid use of uuid module

Upon import of uuid, that module uses ctypes.util to find libuuid/libc,
which involves forking a shell process which then forks and runs ldconfig.

ldconfig performs a whole bunch of disk accesses. This is a slowdown.

uuid.getnode() is also documented to be slow. Lets avoid using it in
the activity launch path; we don't need such a high degree of
randomness.

This makes activities launch 1-2 seconds faster on XO-1.