Werk #6528: TMPFS config hook is now available again

The hook as removed during 1.5 development (#5170). Since it is useful to disable TMPFS especially when Check_MK sites are used in container environments.

E.g. when using Docker tmpfs mounts in containers are only possible with privileged containers or SYS_ADMIN capability. In this situation it is better to turn off the TMPFS in the site and tell docker to mount a tmpfs in the sites tmp directory during container creation (e.g. using docker run --tmpfs /omd/sites/[site]/tmp).

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