Remove positional arguments and replace them with flags

[znerol]

Currently prometheus node exporter accepts the following positional arguments:

positional arguments:
  config                Path to configuration file (pve.yml)
  port                  Port on which the exporter is listening (9221)
  address               Address to which the exporter will bind

Initially pve_exporter had no command line flags at all and positional arguments were the only way to customize the runtime behavior. In the meantime, support for command line flags and environment variables were added. It became possible to run pve_exporter without a config file. Since none of those positional arguments are required, they should be converted into command line flags.

SNMP exporter uses the following flags:

      --config.file="/etc/prometheus/snmp.yml"
                                Path to configuration file.
[...]
      --web.listen-address=:9116 ...
                                Addresses on which to expose metrics and web interface. Repeatable for multiple addresses.