Correctly detect haproxy stats credentials

[javacruft]

Scope grep for stats auth credentials to haproxy configuration
file, rather than the entire /etc/haproxy directory.

See: https://bugs.launchpad.net/charm-helpers/+bug/1713165

[tbaumann]

This changes the grep result. Hence it needs a different undex too.

$ grep -r "stats auth" /etc/haproxy/haproxy.cfg
    stats auth admin:ktTZdNcpKpFYN7Jf42PgVZm5nFJ3zKz3
$ grep -r "stats auth" /etc/haproxy
/etc/haproxy/haproxy.cfg:    stats auth admin:ktTZdNcpKpFYN7Jf42PgVZm5nFJ3zKz3

AUTH=$(grep -r "stats auth" /etc/haproxy/haproxy.cfg | awk 'NR=1{print $4}')

vs

AUTH=$(grep -r "stats auth" /etc/haproxy/haproxy.cfg | awk 'NR=1{print $3}')