Move global path opts in nova.paths

Move the global path config options (i.e. state_path, pybasedir and
bindir) into a new nova.paths module. A new module may seem like
overkill but some utility methods associated with these options follow
in a later commit.

Moving them to nova.paths means they are no longer globally defined
and it's more obvious which modules require these options.

Change-Id: I381d23f1bbe36dc6967a38a65062b0983e1661aa
Merge "Move global path opts in nova.paths"