Set default data_security_mode to "SINGLE_USER" in bundle templates

[anton-107]

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

[denik]

From description:

Change the default Python project template to set data_security_mode of a cluster to SINGLE_USER

This seems to duplicate the title. Could you instead add some background, why is this done, what is going to be the behaviour change for users of the templates, what are the risks.

Change existing unit and integration tests to work with this change

Test descriptions belong in 'Tests' section. It should be specific. if you're relying on existing tests you can omit it or add 'Existing tests'.

From title:

Add default cluster property data_security_mode set to "SINGLE_USER" in default python template

nit: possible shorter title: 'Set data_security_mode to SINGLE_USER in default-python template'

[pietern]

@anton-107 Could you apply the same to the experimental-jobs-as-code template? It applies the same pattern as this template so updating it should be straightforward.

It can be a separate PR.

FYI @kanterov