Add module providing mount scripts for WEKA filesystems

[wiktorn]

Support for mounting WEKA filesystems.

This module doesn't support creating WEKA storage, as this can be done by directly calling [weka-gcp] (https://github.com/weka/terraform-gcp-weka/) module.

Because WEKA require gathering environment information from Metadata server during mount it's not possible to create a fstab entry that will work on following mounts. Hence, the mount is done using SystemD unit.

Things for consideration / cleanup:

• whether to keep network_storage, client_install_runner and mount_runner outputs, or shall those be dropped, to prevent accidental use
• there is quite a few shellcheck disable, because shellcheck doesn't understand terraform templates, to be decided should shellcheck investigate templates as now, or whether add exclusion of template files
• should this PR include a blueprint that provisions the WEKA cluster and uses this module