Adding MySQL validation

@pmorie pls review.
Looked on your code and tried to be a copycat, so hopefully you won't have hard time reading this.
Wasn't sure if the port of the mysql container should be a constant (3306) or it should be taken by its inspection like you did it. So I took the constant way, cause the mysql is using only the 3306 port anyway.

For testing I used : https://github.com/tutumcloud/tutum-docker-mysql image.
The usage is : vdc mysql <containerID> -u username -w password --verbose