SMS monitoring - skip notifications during planned maintenance

[si-the-pie]

Problem

Sometimes servers are temporarily offline during upgrades and installations of new routing edtions.

When these known planned maintenance events occur the SMS should avoid sending notifications.

Proposal

Add detection of the 503 (Service unavailable) header in the tests code.
The failure can then be ignored for specific (end-point) servers but still be sent when the original target was a load balancer.