Add support to configure path to snapshotter agent

[jameshou]

Add ability to configure the path to the snapshotter agent.

We've installed snapshotter in a virtualenv and not at the global system level. As such, cassandra-snapshotter-agent is not globally accessible. And as a result, when we run a backup, the remote command fails because it's unable to find the path to cassandra-snapshotter-agent.

This PR allows us to configure the path to the cassandra-snapshotter-agent when we execute the remote command.

Here is an example of how we can use this new parameter. cassandra-snapshotter --s3-bucket-name=foo_backup_bucket ... --snapshotter-agent-path '... pyenv activate snapshotter-venv cassandra-snapshotter-agent'