When the role is used by grenade, the data directory is shared among
different devstack executions, and the base directory is different,
for example: /opt/stack/data vs /opt/stack/{old,new}.
The new devstack_data_base_dir parameter allows user to specify
a base directory for the data/ directory which is unrelated
to the devstack directory. The default value is devstack_base_dir,
so the default behavior is unchanged.
Change-Id: Ie69b7b51947cbf1a8b31d2701783de2fb56a2d33
3ffc00940f