fix: voting key file provider

This improvement allows devs to use another mechanism for voting key file creating. I would be using this in 2 ways:

1) I have a suite of tests to be pushed that does deep-dir comparison of different setups. The unit test needs a way to generate static files so we can do the assertions on all the files.

The incoming test and unit test file provider: https://github.com/symbol/symbol-bootstrap/blob/networkPresetFile/test/service/BootstrapService.test.ts https://github.com/symbol/symbol-bootstrap/blob/networkPresetFile/test/service/BootstrapService.test.ts#L55

I need other PRs before pushing those unit tests.

2) Symbol services pre-create the voting files when generating the nemesis block on new networks, so they can then be used when creating each node. Those files are stored in the symbol-network's key-store.yml.

fboucquez / symbol-bootstrap

fix: voting key file provider #338