Make devstack grenade code use 'newton' as base branch for migration to
master (ocata).
After switch [1] to dummy driver we need to override defaults.
So, update pre_test_hook approach for saving configuration data properly.
[1] I2b698f69a29ef983b13534b2f4d393570be4dac8
Change-Id: Ie5fa0f78b0e3caa06d34a0822a8ed60b69406f9d
Closes-Bug: #1631412