Added placeholders in default_settings for using S3 as the default file store (strongly recommened for the public AMI).
Added placeholders in default_settings for using S3 as the default file store (strongly recommened for the public AMI).

ehealthafrica-ci/formhub,ehealthafrica-ci/formhub,eHealthAfrica/formhub,eHealthAfrica/formhub,ehealthafrica-ci/formhub,eHealthAfrica/formhub,eHealthAfrica/formhub,ehealthafrica-ci/formhub