Add support for random seed corpus

[jiradeto]

This PR adds an optional flag to enable user to set seed corpora for the experiment. There are two essential components that we need to modify 1.)experiment/run_experiment.py and 2.) experiment/runner.py.

The runner_experiment (run_experiment.py) basically validates the given seed corpus directory and feeds the zipped version of such directory to the fuzzing worker (runner.py). The worker then unzips this directory and randomly pick single input as the initial seed and start the fuzzing campaign.

[jiradeto]

just delete a directory random_seed_corpora_zip that previously used to store corpora temporarily. Now we have the following structure:

|— my_corpus
|    |— sqlite3_ossfuzz  
|    |    |— fbeb1de0be72d83ed8dff924fd9df8cedb0a7497
|    |    |— fba1b82f6612e8e1d1786b757dfeecbbfabeeca5
|    |— libjpeg-turbo-07-2017
|    |    |— f78602c270b787d3e7e940f50f78365552d5a20b
|    |    |— ebc89a181d637f6bdfdcaa9d647cfd1f88d62675
|    |    |— ed0fbdb5c9f7d2fbb1c6c8e0665e80b110ba1e14
|    |— sqlite3_ossfuzz.zip
|    |    |— fbeb1de0be72d83ed8dff924fd9df8cedb0a7497
|    |    |— fba1b82f6612e8e1d1786b757dfeecbbfabeeca5
|    |— libjpeg-turbo-07-2017.zip
|    |    |— f78602c270b787d3e7e940f50f78365552d5a20b
|    |    |— ebc89a181d637f6bdfdcaa9d647cfd1f88d62675
|    |    |— ed0fbdb5c9f7d2fbb1c6c8e0665e80b110ba1e14

[jiradeto]

Closing this because we have to split into separated options. One for custom corpus (#3) and another for random seed selection.