Don't show containers in deployment summary.

* Filter out container only changes in the `_addMachines` case of
`_getChanges`
* Added `_noMajorChanges`, which determines if there are any so-called "major"
 changes in a given change set.
* Updated the handlebars template to show the summary only for "major"
* and to toggle the changelog by default if there are only "minor" changes.
* Updated tests.
The falsy property name here and the `unless` in the template is a little mind warping, could this be inverted to be truthy?