Add xnat_link_data_directory variable for XNAT role (#3)

- add `xnat_link_data_directory` variable for `xnat` role (defaults to
`true`)
- there is already a `xnat_create_data_directory` that if `true` will
ensure `xnat_web_server.storage_dir` exists with `tomcat` as the owner,
and then creates a symbolic link to `external_storage_drive`
- however, we need to distinguish between creating the data directory
and linking to external storage
- if `external_storage_directory` is defined, and is the same path as
the XNAT storage directory, then we musn't create and set permissions
for this path in the XNAT role. If we did, the mounted storage will have
the incorrect permissions
- so, if the `external_storage_drive` is the same path as
`xnat_web_server.storage_dir`, then the following values should be set:
```yaml
xnat_create_data_directory: false # don't set permissions for the mounted storage
xnat_link_data_directory: true
```

roles/xnat/defaults/main.yml

@@ -13,3 +13,4 @@ xnat_config_dir: "{{ xnat_home_dir }}/config"
 xnat_logs_dir: "{{ xnat_home_dir }}/logs"
 xnat_work_dir: "{{ xnat_home_dir }}/work"
 xnat_create_data_directory: true
+xnat_link_data_directory: true

roles/xnat/tasks/directories.yml

@@ -13,7 +13,7 @@
     src: "{{ xnat_web_server.storage_dir }}"
     dest: "{{ xnat_data_dir }}"
     state: link
-  when: xnat_create_data_directory and external_storage_drive is defined
+  when: xnat_link_data_directory and external_storage_drive is defined
 
 - name: "Ensure XNAT subdirectories exist"
   ansible.builtin.file: