Set a sane default for state_path

This change sets a sane default to state_path and puts the previous
state_path for tests only.

Because the default value of state_path changed, if this one isn't set in the
configuration file, pybasedir is not more used as replacement but /var/lib/nova
is used instead.

UpgradeImpact
DocImpact
Closes bug: #1240879

Change-Id: I94502bcfac8b372271acd0dbc1710c0e3009b8e1
Merge "Set a sane default for state_path"