(PUP-2571) add 'before' functionality to file_line

file_line supports adding lines after a match, but there are use cases when
having "before" would be useful. For example, in Debian-based OS's, the last
line of /etc/rc.local is "exit 0" it's an incredible pain to deal with
that scenario today.

This commit adds a 'before' parameter to the file_line type, and implements
it for the ruby provider.