vmalert to support notifier config

[hagen1778]

Support of https://docs.victoriametrics.com/vmalert.html#notifier-configuration-file is supposed to simplify notifiers configuration process. There will be no need to backport flag values anymore, since all of them can be configured via file.

[hakuno]

It's even needed for setting up dns_sd_configs.
The current Helm Chart can't support that.

[hakuno]

@hagen1778 I'm sure it solves #572

[Haleygo]

Now users can use -notifier.config=</path/file> in following steps:

1. use extraObjects to create extra configmap or secret;
2. mount configmap/secret to /path using extraVolumes.

[Haleygo]

Close as completed.