ceph-mixin: fix manually edited 'prometheus_alerts.yml' file

ceph-mixin: fix manually edited 'prometheus_alerts.yml' file

File 'prometheus_alerts.yml' file should not be edited directly.
The changes should be added to 'prometheus_alerts.libsonnet' file
(and/or any other appropriate lib/j sonnet files) and generated
using 'make generate' command.

Adding all the changes to 'prometheus_alerts.libsonnet' file and
building/generating the prometheus_alerts YAML file.

PS: all the changes seen in 'prometheus_alerts.yml' file is due
to the re-arrangement of lines. The file remains same.

Signed-off-by: Arun Kumar Mohan <amohan@redhat.com>