rework namespace to not live in nova's namespace

Essentially, this puts us into top level namespace of 'ncflex'
rather than nova.  It means that instead of 'compute_driver' of
'nova.flex.virt.LXCDriver' we have 'ncflex.nova.virt.flex.LXCDriver'.

This is similar to how ironic was namespacing their driver.

Also here we change to use relative imports.