[alertmanager] Route InfoInhibitor alert to a null receiver

This alert is regularly fired on the sysadm irc room:

11:31 -- Notice(swhprombot): Alert NONE firing - admin/cluster-admin - InfoInhibitor - This is an alert that is used to inhibit info alerts.
11:31 -- Notice(swhprombot): By themselves, the info-level alerts are sometimes very noisy, but they are relevant when combined with
11:31 -- Notice(swhprombot): other alerts.
11:31 -- Notice(swhprombot): This alert fires whenever there's a severity="info" alert, and stops firing when another alert with a
11:31 -- Notice(swhprombot): severity of 'warning' or 'critical' starts firing on the same namespace.
11:31 -- Notice(swhprombot): This alert should be routed to a null receiver and configured to inhibit alerts with severity="info".
11:32 -- Notice(swhprombot): Alert NONE resolved - admin/cluster-admin - InfoInhibitor - This is an alert that is used to inhibit info alerts.
11:32 -- Notice(swhprombot): By themselves, the info-level alerts are sometimes very noisy, but they are relevant when combined with
11:32 -- Notice(swhprombot): other alerts.
11:32 -- Notice(swhprombot): This alert fires whenever there's a severity="info" alert, and stops firing when another alert with a
11:32 -- Notice(swhprombot): severity of 'warning' or 'critical' starts firing on the same namespace.
11:32 -- Notice(swhprombot): This alert should be routed to a null receiver and configured to inhibit alerts with severity="info".