To make sure we can find the yaml warning before code merge[1].
We can enable strict mode to treat warning as error[2].

[1] https://review.openstack.org/#/c/573482/1
[2] http://yamllint.readthedocs.io/en/stable/configuration.html#errors-and-warnings

Change-Id: I0b54bc2bd384f092f2fcd1b0396e133a0d95befe