alertmanager: checkpath to ensure storage dir

The last commit was a red herring, the actual problem is that the
storage dir was not there.

Revert "alertmanager: remove storage path option"

This reverts commit f37fd9e4ce5fa86699cb8e2391f8fe8064d67c44.

alertmanager: remove storage path option

This was a copy+paste error from prometheus.

alertmanager: normalize initd/confd

Move prometheus packages into main repo

prometheus: checkpath on log files

Update prometheus, alertmanager init scripts

Normalize alertmanager package

prometheus/alertmanager: new APKBUILD