swift: fix swift::proxy::monitoring for eqiad

First of all, check the well-known /monitoring/backend instead of
/wikipedia/commons/thumb/a/a2/Little_kitten_.jpg/80px-Little_kitten_.jpg
which may or may not exist (duh!)

Second, check with the respective address for each cluster and remove
the silly check_http_swift ad-hoc check but use the standard
check_http_url.

Third, let's add checks for the proxies (/monitoring/frontend) while
we're at it as to ease future troubleshooting.

Change-Id: I4b38eb14b7a6861259cf3ee3c118b2730a775fde