Added functionality to 'await' successful creation of an image or a snapshot.

:information_source: This is a living pull request, the initial commit covers the library functionality and the straight forward 'await' handling for `backup-(instances|volumes)` scripts; pending parts are:

1. adding 'await' handling to `create-(images|snapshots)` scripts (might actually obsolete the `backup-*` scripts again)
1. adding separate `await-(images|snapshots)` scripts to allow for granular usage/reporting, e.g via a Jenkins job pipeline