Introduce some interactivity to artifact cache expiry

[See original issue on GitLab](https://gitlab.com/BuildStream/buildstream/-/issues/490)
In GitLab by [[Gitlab user @tlater]](https://gitlab.com/tlater) on Jul 23, 2018, 10:17

## Background

[//]: # (Provide a background or the root/source that justifies this task or action.)

Currently, when a build pipeline does not fit inside the artifact cache quota, buildstream will unconditionally give up. This is alright, but perhaps annoying to the user.

## Task description

In interactive mode, buildstream should prompt the user, allowing them to temporarily increase the disk size if they want to - this should have three options:

- Abort the build, keeping all artifacts (default)
- Abort the build, purging some artifacts used in this build
- Continue building, ignoring artifact cache quota for this build

## Acceptance Criteria

We have a nicer UI for running out of cache space.

[//]: # (Acceptance criteria should follow the S.M.A.R.T. principle https://en.wikipedia.org/wiki/SMART_criteria )

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The issue names no files, tests, or entry points. Start by tracing artifact-cache quota handling and the interactive-mode flow, then implement and verify the three specified choices when the cache quota is exceeded; done means interactive builds provide the described cache-space behavior.

Written by the indexing model from the issue text.