SE-3222 Extend EDXAPP_SITE_CONFIGURATION with default values

Created by: s0b0lev

This PR introduce new merge function for EDXAPP_SITE_CONFIGURATION settings.

Testing instructions This instruction is meant to be run locally:

1. Create a new OpenEDXInstance
2. Open OCIM django shell make shell
3. Get created instance:

>>> instance = OpenEdXInstance.objects.last()

4. Create new appserver (without spawning actual deployment, we are care only about app server final configuration)

>>> instance._create_owned_appserver()

5. Open UI, check that Combined ansible vars -> EDXAPP_SITE_CONFIGURATION looks like this:

EDXAPP_SITE_CONFIGURATION:
- values:
    CONTACT_US_CUSTOM_LINK: /contact

6. Open instance settings (django admin page http://localhost:5000/admin/instance/openedxinstance/)
7. Update Configuration extra settings:

DEMO_CREATE_STAFF_USER: false
SANDBOX_ENABLE_CERTIFICATES: false
demo_test_users: []
EDXAPP_SITE_CONFIGURATION:
- values:
    CONTACT_US_CUSTOM_LINK: /override-contact-us-link
    TEST_VAR: /test

8. Repeat step 4
9. Open UI, check that Combined ansible vars -> EDXAPP_SITE_CONFIGURATION looks like this:

EDXAPP_SITE_CONFIGURATION:
- values:
    CONTACT_US_CUSTOM_LINK: /override-contact-us-link
    TEST_VAR: /test

Possible edge scenarios to verify:

• Appserver provisioned with default EDXAPP_SITE_CONFIGURATION
• Appserver provisioned with additional Configuration extra settings
• Appserver prodivioned with overridden default for EDXAPP_SITE_CONFIGURATION (for example: CONTACT_US_CUSTOM_LINK)

Author notes @nizarmah I am sorry for such large instruction, but this is the only way to verify it works. Feel free to ask any questions or clarify anything you need for this review.

Thanks!