Don't assume all numbers are float in boxgrinder-build CLI

While running boxgrinder-build, if you pass in some options like `--delivery-config host:1.1.1.1`, it will be evaluated as `@plugin_config['host'] = 1.1`. The expected result would be `@plugin_config['host'] = '1.1.1.1'`. This is due to a small bug in a regular expression used to convert strings into floats in the boxgrinder-build CLI interface.

The fix involves looking for sane numbers only, meaning:

* 1 is a number
* 1.1 is a number
* 1.1e+10 is a number
* 1.1.1 is a string

This patch ensures that at most one decimal is provided before float conversion happens.