BZ#1168433 - undefined method `to_a' for "ens7":String

https://bugzilla.redhat.com/show_bug.cgi?id=1168433

We want to allow both a string and an array of strings as the
`the_nic` parameter to `find_ip` function. However, `to_a` method is
not defined on strings. We now use a different approach with `flatten`
method.