Ease to switch themes of Horizon

Ease to switch themes of Horizon

JIRA: -

Default Horizon theme is ubuntu. If you want to use openstack theme,
you can run `export ENABLE_UBUNTU_THEME=False` before deploy.

And if you want to change theme after you installed openstack, you need
to run this commands:
  1. access compass VM, password is root
      `ssh root@192.168.200.2`
  2. change path:
      `cd /var/ansible/run/openstack_newton-opnfv2`
  3. Edit the `group_vars/all` file, change `enable_ubuntu_theme` var:
      `enable_ubuntu_theme: False`
     if `enable_ubuntu_theme: True`, the Horizon theme is ubuntu, otherwise
     the Horizon theme is openstack.
  4. run ansible playbook:
       `ansible-playbook -i inventories/inventory.yml HA-ansible-multinodes.yml
        -t horizon_theme`

Change-Id: I5465107228af32a482879c8a058899494f33e2d0
Signed-off-by: liyuenan <liyuenan@huawei.com>