Provide multiple backends

[mssola]

zypper-docker should be able to support not only zypper, but other package management systems as well. This way we will broaden the scope of the project. This issue involved the following:

• Design and implementation of a backend system.
• Implementation of another backend besides zypper (e.g. dnf or apt).

See the existing cli-separation branch.
Depends on #125.