Add API to update openrc variable

Add API to update openrc variable

JIRA: YARDSTICK-651

Currently we source openrc variable manually or use yardstick env
prepare to get openrc file.
We need API to update the openrc variable.

api: /yardstick/env/action
description: source environment variable
method: POST

parameters:
{
    'action': 'update_openrc',
    'args':{
        'openrc': {
            'OS_USERNAME': 'admin',
            'OS_PASSWORD': 'console',
            'OS_TENANT_NAME': 'admin',
            'OS_AUTH_URL': 'http://192.168.131.222:5000/v2.0',
            'EXTERNAL_NETWORK': 'ext-net'
        }
    }
}

Change-Id: I680a7249116c8ff0c1a9e7d5089538d935240c80
Signed-off-by: chenjiankun <chenjiankun1@huawei.com>