Fixes #3847: lineinfile without regex

I also wanted to manage known_hosts with lineinfile, and this change Works For Me(tm).  I couldn't think of a decent change for the help text, so I didn't make a change there.  I can think about it more if you won't merge without it. :)

Description from #3847 ...

We'd like to use lineinfile to add entries into our ssh_known_hosts file.
Unfortunately we can't use the line itself as the regex value, because it contains non-escaped characters.
What would be great is you could omit the regex option, and then ansible would match only the insertion text verbatim.