Don't allow rpk redpanda start to run as root without an over ride flag

[rkruze]

Currently, if you run rpk redpanda start as root on a node that has the redpanda service installed and configured via systemctl then it will overwrite the config files to be writable only by root and also created several files and folders in the data directory that are owned by root. To avoid this, we should throw an error if you are trying to run rpk redpanda start as root unless you specify a --run-as-root or something similar.

[esteban]

Overwriting configs is bad, shouldn't we generate configuration files only when is explicitly indicated by the operator instead?

[rkruze]

@0x5d can comment, but the framework we use normally rewrites the config when changes or things are not in order.

[0x5d]

@esteban Some flags, like --kafka-addr, --rpc-addr, etc., can't be passed to redpanda through the CLI when rpk execs into redpanda. Therefore the only way to relay those config values is to update the config file, so that redpanda picks them up when it starts.