Add server_name_prefix semi-random naming with some known prefix

Of the form

<SERVER NAME PREFIX>-<RANDOM STRING>

for example

myproject-asdfghjk
Fix random string concatenation bug and add extra test for blank server_name_prefix
Fix Rubocop warning
Merge pull request #63 from ftclausen/server_name_prefix

Add server_name_prefix semi-random naming with some known prefix