Abstract common methods and classes from DockerBackend

[dhodovsk]

Recently, there were added multiple features to DockerBackend (e.g. https://github.com/fedora-modularity/conu/commit/2a4a407a237093b877e95fa6c6705269f1a24bc2 ) We should gather the new features and when it makes sense, move them to stable API, so they can be used also by other backends.

@jscotka Please add some examples for the features if you have.

[jscotka]

for example methods like:

• __repr__
• __str__
• inspect
• list_all
• cleanup for list all of them, list all TODOs comments in nspawn section with text like API

or class like ImagePullPolicy are very generic and move part of init with this policy to Imageclass init

or some constants, like CONU_ARTIFACT_TAG, in nspawn I've also defined this

[dhodovsk]

First step for doing this is research of existing backend and creation of list of methods that can be used in absract class