It obtains a valid token.
    :param url_base: keystone url
    :param tenand_id: the id of the tenant
    :param user: the user
    :param paassword: the password

def get_token(url_base, tenant_id, user, password):
    """It obtains a valid token.
    :param url_base: keystone url
    :param tenand_id: the id of the tenant
    :param user: the user
    :param paassword: the password
    """
    url = 'http://' + url_base + '/v2.0/tokens'
    headers = {'Content-Type': 'application/json'}
    payload = '{"auth":{"tenantId":"' + tenant_id +\
              '","passwordCredentials":{"username":"'\
              + user + '","password":"' + password + '"}}}'
    response = requests.post(url, headers=headers, data=payload)
    if response.status_code != 200:
        print 'error to obtain the token ' + str(response.status_code)
        exit(1)
    response_json = response.json()
    token = response_json['access']['token']['id']
    return token