Add a expansion functionality

Add a expansion functionality

Through modify the virtual_cluster_expansion.yml, include
host's name and mac, you can decide to how many compute
nodes you need to add.

And you also need to modify network.yml. Note that external
subnet's ip_range should be changed as the first 6 IPs are
already taken by the first deployment.

Edit ``add.sh``, check the environment variable. Note that
the OS version and OpenStack version should be same as the
first deployment.

Run ``add.sh``.

JIRA:COMPASS-481

Change-Id: Id85f02518667e0ff80c2475e70856cd30cf1b9b7
Signed-off-by: liyuenan <liyuenan@huawei.com>