nixos/prometheus: replace 'alertmanagerURL' options for prometheus2

Prometheus2 does no longer support the command-line flag to specify
an alertmanager. Instead it now supports both service discovery and
configuration of alertmanagers in the alerting config section.

Simply mapping the previous option to an entry in the new alertmanagers
section is not enough to allow for complete configurations of an
alertmanager.

Therefore the option alertmanagerURL is no longer used and instead
a full alertmanager configuration is expected.