nminit.py should read user keys from command line

[aaaaalbert]

nminit.py is a helper script that creates configuration files required for running the nodemanager. It is usually employed in a local (non-clearinghouse/custominstallerbuilder) setup, e.g. for development and testing purposes. Currently, the owner/user keys it will plant in the vessels are hardcoded.

My proposal is to not hardcode the user keys, but make nminit take command line options specifying the files from which public keys should be read. These keys are then made the owner keys of the created vessels.

Newcomers: Additional documentation on the nodemanager and vessels is available here. We have libraries to read in user key files in the appropriate format.