This patches update the devstack to automatically
set the new tempest configuration `boot_mode`,
it will use the value from IRONIC_BOOT_MODE variable.
Increase the number of VM's in ironic-tempest-ipa-partition-pxe_ipmitool
and ironic-tempest-ipa-wholedisk-bios-agent_ipmitool-tinyipa
to 2 since it runs cleanning and now we run two tempest tests.
Depends-On: https://review.opendev.org/735960
Change-Id: Ic6faf73430e56e2b1ff19a72b1b03f8ef34eff5f