Add alermanager persistant storage

[AdamNuclear]

Please add a possibility to assign a mount for persistent storage to alert-manager container.
This can make silences survive a restart of monitoring which is useful to prevent false alerts on clusters where is maintenance in progress.

We already have implemented passing commands to alertmanager:

  -C alertmanager commands       - Pass the command to the alertmanager.

Would be great to have the possibility to pass also docker mounts.