Updated armband

Updated armband
Project: yardstick  fe46800c17c055ee337dddb2e2de00d6db128cda

Bugfix: Only add keyword "external_network" to the first network

In test case yaml file, when vm need three networks and enable
floating ip true, it should only add keyword "external_network"
to the first one.

'networks': {'test': {'cidr': '10.0.1.0/24', 'external_network':
'admin_floating_net'}, 'test3': {'cidr': '10.0.3.0/24'},
'test2': {'cidr': '10.0.2.0/24'}}

JIRA:YARDSTICK-309

Change-Id: I1126955749b598b8c5460ac75843c1939b4f8443
Signed-off-by: wu.zhihui <wu.zhihui1@zte.com.cn>