Improve tempest config generation for all main OpenStack configurations

Rally tempest integration should auto-generate well-working config for with all main OpenStack configurations.

The configurations for testing are obtained from
https://github.com/openstack-infra/config/blob/master/modules/openstack_project/files/zuul/layout.yaml

To generate DevStack config files, use the script gen_configs.sh given below (most of code is from github.com/openstack-infra/devstack-gate/).

Get the script: https://raw.githubusercontent.com/maretskiy/var/master/devstack/gen_configs.sh

Deployment workflow:
1. ./unstack.sh
2. edit gen_configs.sh:
  * put deployment shell code into section CUSTOM CONFIG
  * set authentication via environment variable DEVSTACK_AUTH
3. run script to get new localrc and exerciserc
4. ./stack.sh

Here are the DevStack configurations and shell scriptlets to apply them:

  check-tempest-dsvm-neutron-pg
          export DEVSTACK_GATE_TIMEOUT=120
          export DEVSTACK_GATE_TEMPEST=1
          export DEVSTACK_GATE_POSTGRES=1
          export DEVSTACK_GATE_NEUTRON=1
          export DEVSTACK_GATE_CONFIGDRIVE=0
          export DEVSTACK_GATE_SMOKE_SERIAL=1

  check-tempest-dsvm-neutron-heat-slow
          export DEVSTACK_GATE_TIMEOUT=60
          export DEVSTACK_GATE_TEMPEST=1
          export DEVSTACK_GATE_NEUTRON=1
          export DEVSTACK_GATE_TEMPEST_HEAT_SLOW=1

  check-tempest-dsvm-large-ops
          export DEVSTACK_GATE_TIMEOUT=60
          export DEVSTACK_GATE_TEMPEST=1
          export DEVSTACK_GATE_TEMPEST_LARGE_OPS=1

  check-tempest-dsvm-neutron-large-ops
          export DEVSTACK_GATE_TIMEOUT=60
          export DEVSTACK_GATE_TEMPEST=1
          export DEVSTACK_GATE_TEMPEST_LARGE_OPS=1
          export DEVSTACK_GATE_NEUTRON=1

  check-tempest-dsvm-cells
          export DEVSTACK_GATE_TIMEOUT=60
          export DEVSTACK_GATE_TEMPEST=1
          export DEVSTACK_GATE_CELLS=1

  check-tempest-dsvm-cells-full
          export DEVSTACK_GATE_TIMEOUT=120
          export DEVSTACK_GATE_TEMPEST=1
          export DEVSTACK_GATE_TEMPEST_FULL=1
          export DEVSTACK_GATE_CELLS=1