Set default to false for nova v3_api config option

This commit switches the default value for the nova v3 api config
option from true to false. This changes the default behavior to not
run the Nova v3 API tests. This is because the v3 is still marked as
experimental and is able to change in a non-backwards compatible way
which will cause issues when running with branchless tempest.
Additionally, the nova team decided not to do a standalone v3 api so
at some point this will disappear and instead become v2.1 of the api
with microversions. When you couple this with the fact that tempest's
gate budget is basically exhausted (tests shouldn't take over an hour
to run in the gate) this should be defaulted off. For more discussion
see:

http://lists.openstack.org/pipermail/openstack-dev/2014-June/037370.html

Change-Id: I13dbc4b433a0f6defbeebce958b2e1cbcf64ba7a
Merge "Set default to false for nova v3_api config option"