(MAINT) Feature/curl retries

@anodelman 

`CONFIG` is merged into the `host_options` hash here:
https://github.com/puppetlabs/beaker/blob/master/lib/beaker/options/hosts_file_parser.rb#L33

That hash gets merged into the options file hash here:
https://github.com/puppetlabs/beaker/blob/master/lib/beaker/options/parser.rb#L203

My analysis could be wrong, but I believe the host options are merged with the
global state options here:
https://github.com/puppetlabs/beaker/blob/master/lib/beaker/host.rb#L61

So therefore the `curl-retries` can be defined in either the options file, the
CONFIG hash in the host config file, or in the host-specific section of the host
config file and it will still be accessible via `host['curl-retries']` without
having to fall back to the actual `options` global hash.