support host_pattern in config devices list

[matejv]

Adds ability to specify host_pattern: True in configuration file under devices. In that case the host value is treated as a regex.

That allows you to not have to specify every target in exporter configuration, but to have prometheus server scrape dynamically added or removed hosts.

When scraping metrics if no target query parameter is given in URL, any device that has host_pattern: True is skipped. However if target is supplied, and it doesn't match any host values exactly, then host_pattern entries are compared.

The code is mostly the same as in https://github.com/czerwonk/junos_exporter