Fixing gentoo init script to use system python.

If this is custom compiled on Gentoo (like it should be), the built in
python can have problems being found (especially if security mechanisms
are put in place (i.e. pax)).  This change tells nova-agent to use the
system python and check the system python's path before looking for the
nova-agent specific modules in the nova "shared" directory.
Merge pull request #1 from alunduil/gentoo-init-fixes

Fixing gentoo init script to use system python. Requires update for NOVA_PYTHON_PATH evaluation to depend on nova-agent's internal Python version and not on the system where it's being installed. Will push that soon.