Set default data_security_mode to "SINGLE_USER" in bundle templates (#2372)

## Changes
1. Change the **default-python** bundle template to set
`data_security_mode` of a cluster to SINGLE_USER
2. Change the **experimental-jobs-as-code** bundle template to set
`data_security_mode` of a cluster to SINGLE_USER

## Why
Explicitly adding this field saves experienced users from confusion onto
what security mode is applied to the cluster

## Tests
Changed existing unit and integration tests to pass with this change

acceptance/bundle/templates/default-python/output/my_default_python/resources/my_default_python.job.yml

@@ -44,6 +44,7 @@ resources:
           new_cluster:
             spark_version: 15.4.x-scala2.12
             node_type_id: i3.xlarge
+            data_security_mode: SINGLE_USER
             autoscale:
                 min_workers: 1
                 max_workers: 4

acceptance/bundle/templates/experimental-jobs-as-code/output.txt

@@ -34,6 +34,7 @@ Warning: Ignoring Databricks CLI version constraint for development build. Requi
               "max_workers": 4,
               "min_workers": 1
             },
+            "data_security_mode": "SINGLE_USER",
             "node_type_id": "i3.xlarge",
             "spark_version": "15.4.x-scala2.12"
           }

acceptance/bundle/templates/experimental-jobs-as-code/output/my_jobs_as_code/resources/my_jobs_as_code_job.py

@@ -56,6 +56,7 @@ my_jobs_as_code_job = Job.from_dict(
                 "new_cluster": {
                     "spark_version": "15.4.x-scala2.12",
                     "node_type_id": "i3.xlarge",
+                    "data_security_mode": "SINGLE_USER",
                     "autoscale": {
                         "min_workers": 1,
                         "max_workers": 4,

integration/bundle/testdata/default_python/bundle_summary.txt

@@ -82,6 +82,7 @@
                 "max_workers": 4,
                 "min_workers": 1
               },
+              "data_security_mode": "SINGLE_USER",
               "node_type_id": "i3.xlarge",
               "spark_version": "15.4.x-scala2.12"
             }

libs/template/templates/default-python/template/{{.project_name}}/resources/{{.project_name}}.job.yml.tmpl

@@ -69,6 +69,7 @@ resources:
           new_cluster:
             spark_version: {{template "latest_lts_dbr_version"}}
             node_type_id: {{smallest_node_type}}
+            data_security_mode: SINGLE_USER
             autoscale:
                 min_workers: 1
                 max_workers: 4

libs/template/templates/experimental-jobs-as-code/template/{{.project_name}}/resources/{{.project_name}}_job.py.tmpl

@@ -97,6 +97,7 @@ This job runs {{.project_name}}_pipeline on a schedule.
                 "new_cluster": {
                     "spark_version": "{{template "latest_lts_dbr_version"}}",
                     "node_type_id": "{{smallest_node_type}}",
+                    "data_security_mode": "SINGLE_USER",
                     "autoscale": {
                         "min_workers": 1,
                         "max_workers": 4,