keep multiple traces

[nikaro]

Currently this plugin keep only one trace.json, thus not allowing to run multiple plays and have the trace for each of them.

I suggest to :

• change the default output directory to ./trace (or something else) to avoid cluttering the current dir with too many files
• change the output file to something like play-<timestamp>-<playbook_name>.json
• add an option to set a rotation for these trace files (ie. keep only 30 of them)

I already have proof of concept locally, if you are OK with i can submit a pull request that we can discuss further.