SE-3629 Store MKTG_URL_LINK_MAP in EDXAPP_MKTG_URL_LINK_MAP

Created by: s0b0lev

This merge request is about resolving an issue with MKTG_URL_LINK_MAP override.

Testing instructions

• Create a new instance
• Create new appserver
• Verify that key EDXAPP_MKTG_URL_LINK_MAP in Combined ansible vars: exists
• This key should contain all default pages
• Set yaml to the configuration_extra_settings

EDXAPP_MKTG_URL_LINK_MAP:
  HONOR: !!null
  BLOG: !!null
  DONATE: !!null

• Create new appserver
• Check that key EDXAPP_MKTG_URL_LINK_MAP in Combined ansible vars: was updated in regards to configuration_extra_settings

Author's notes and concerns

• These changes will work for already existed instances

Reviewers

• @nizarmah