Add API to update hosts info about SUT

Add API to update hosts info about SUT

JIRA: YARDSTICK-674

For some SUT which use domain name as endpoint and AUTH_URL, yardstick
restAPI should support to add "ip domain_name" info into /etc/hosts so
that it can use the ip_address to access the SUT.

api: /yardstick/env/action
description: update hosts info for domain name
method: POST

parameters:
{
    "action": "update_hosts",
    "args": {"opnfv.org": "1.1.1.1",
             "openstack.org": "2.2.2.2"
            }
}

Change-Id: Iee1defaf069ebcb0328a37a659024eabac297482
Signed-off-by: rexlee8776 <limingjiang@huawei.com>