Add test scenaro for atomic mount and unmount testing

[chuanchang]

As subject.

[miabbott]

@chuanchang I have a couple of comments/questions inline, but overall this looks good. Thanks for the additional tests!

[chuanchang]

@miabbott, thanks for your nice comments, the v2 is comming.

[miabbott]

@chuanchang In steps/containers_kube.py, many of the remote_cmd() calls use remote_user=root. Some of these were originally there, while others you have added in your new methods.

Could you test if removing the remote_user argument in your methods will still cause them to work properly?

[chuanchang]

@miabbott, I double checked 'remote_user=root' argument in my real testing work, it's not necessary for testing, so I have removed them from steps/containers_kube.py in my v2 patches.

[chuanchang]

v2 ~ v1

containers_kube: 1. add new functions to get images id
                 2. remove unused remote_user="root" argument

atomic: 1. update codes according previous comments
        2. add new test scenario for {m|unm}ount image to specified directory

[miabbott]

:+1: