[bug] Default template selection must filter by backend

[nemesifier]

Try creating a template flagged as default for the OpenWRT backend, then create a new device, click on "add configuration", then select OpenWISP Firmware 1.x as backend, then select the organization.

Expected result: the default template for the OpenWRT backend is not auto selected.

Actual result: the default template for the OpenWRT backend is selected.

We must modify the logic of the device admin page so that:

• the value of the backend field can be passsed and used as a filter (must be required), please ensure the interaction with the method Config.get_default_templates respects the backend filtering already done by that method
• the JS logic should listen also to the on change of the backend field, it must trigger the HTTP request only when both organization and backend are selected
• we should also add a new test in config.tests.test_controller to ensure the backend is respected: create a default template for openwrt, register a device using the openwisp1.x backend and ensure the default template for openwrt is not assigned. This should work according to the implementation of Config.get_default_templates