Rewrite fqdn_rand_string() as a Puppet 4.x function

[smortex]

The 3.x function rely on is_integer() which is deprecated. Rewrite it using the more modern puppet 4.x function to rely on data types for better parameters validation.

• Rework the tests to check actual values;
• Rewrite fqdn_rand_string() as a Puppet 4.x function.

[alexjfisher]

IMO all non core puppet functions should be namespaced. My approach has been to create a non-namespace deprecated shim that we can remove at a later date.

[smortex]

@alexjfisher hum, yeah, maybe we can do this. For now, I want to remove the deprecated functions so need a backward compatible way to use fqdn_rand_string() in order to open another PR I am preparing ATM.

When these are merged, we can namespace the function and add a deprecation warning in the non-namespaced version in another PR if you are okay with this?