Only render pipeline graphs initially, and then when all proofs are completed

[karkhaz]

Currently, Litani renders the dependency graphs for each pipeline every time it renders a report. This means that it forks off an invocation of dot for every pipeline, every time the report is re-generated. This is a huge waste of comput time.

Acceptance:

• Litani should only render the pipeline dependency graphs twice. Once at the beginning of the run; it should then re-use the SVGs for subsequent renderings of the report. Litani should then re-render the dependency graphs after the run completes, at which point the nodes will change colour depending on whether the jobs were successful.