Fix errors in lookup plugins.

Lookup plugins 'sequence' and 'template' now import 'ansible.utils'
appropriately in order to use the 'listify_lookup_plugin_terms'
function.

Also, 'dnstxt' and 'env' now check to see if 'terms' is a string;
without this calls like '{{ lookup('env', 'HOME') }}' fail.

I'm not 100% positive why the latter change is needed, as 'listify_lookup_plugin_terms' *seems* to replace a string with a single-item list. I didn't look that hard though.