(cherry picked from commit 207c5e0f)

* the CMS defaults to PRIMARY
* the LMS defaults SECONDARY_PREFERRED, except for the draft docstore.
* Removes EDXAPP_CONTENTSTORE_ADDITIONAL_OPTS

and the LMS split docstore read_preference to `SECONDARY_PREFERRED`.

to indicate that this is the default read_preference for all mongo
replicasets.

Adds EDXAPP_LMS_DRAFT_DOC_STORE_READ_PREFERENCE, so it can differ from
the EDXAPP_MONGO_READ_PREFERENCE and default to PRIMARY.

Updates the CHANGELOG to indicate the variable changes, and to better
clarify their purpose and usage.

Adds 'trashcan' in its place, because it is.

and inserts it into the modulestore and contentstore configuration blocks.

Delete edxapp_migrate.yml

I'm not sure where we use this and I was unable to find any references to it anywhere.

add default variable DEFAULT_MOBILE_AVAILABLE

NIT: Update the task name.  Not adding a changelog entry since this is just cosmetic.

Fix EDXAPP_CELERY_BROKER_TRANSPORT OPS-2438

Add support for LMS_INTERNAL_ROOT_URL.

This adds ability to configure the LMS_INTERNAL_ROOT_URL setting via the
EDXAPP_LMS_INTERNAL_ROOT_URL ansible variable. The value of the new
variable is used instead of EDXAPP_LMS_ROOT_URL to construct enterprise
API URLs, which are used for internal communication. In some deployment
scenarios, it is not desirable for internal communication to be carried
out via the external URLs.

This change is backwards compatible since the new EDXAPP_LMS_INTERNAL_ROOT_URL
defaults to the value of EDXAPP_LMS_ROOT_URL.

OPS-2438 Add support for redis as Celery backend

Add read preference for module store OPS-2440

Enable HTTP Strict Transport Security even behind the ELB

Merge pull request #4204 from edx/tasawer/learner-2736/add-oauth-access-token-for-zendesk-integration-updated

Update var name "ZENDESK_OAUTH_ACCESS_TOKEN"

splunk changes for edx

LEARNER-2736

Allow 'automated' and 'edx_service' roles to run multiple times.

Merge pull request #4202 from edx/tasawer/learner-2736/add-oauth-access-token-for-zendesk-integration

Add oauth access token for Zendesk integration

Revert "Add mongo read preference OPS-2440"

This reverts commit a72becb1.

LEARNER-2736

Add mongo read preference OPS-2440

Set a default language for the notifier.

Upgrade junit and cobertura jenkins plugins