Allow either foo.bar or foo.bar.production style names for nagios

Nagios hosts must be specified by fqdn (or at least, by the name
specified in the nagios configuration file). Shorter names are silently
ignored by the nagios process. However, many of our other fabric
commands are used with shorter names.

This commit allows foo.bar-style names as well as foo.bar.production. It
also makes the fabric task fail noisily if it doesn't understand the
hostname (which likely indicates that neither would nagios).
Merge pull request #18 from alphagov/flexible-nagios-hostnames

Allow either foo.bar or foo.bar.production style names for nagios