Output of slack.post_message not parsing '\n'

SUMMARY

Code snippets:

# workflows/list_instances.yaml
      finish:
        action: centro.post_message
        input:
          channel: <% $.channel %>
          module_name: <% $.module_name %>
          environment: <% $.environment %>
          execution_id: <% $.execution_id %>
          user: <% $.user %>
          message: "```<% $.instance_list %>```"
        on-complete:
          - succeed

# chains/post_message.yaml
---
  chain:
    -
      name: "post_message"
      ref: "slack.post_message"
      parameters:
        username: "marvin"
        channel: "{{ channel }}"
        icon_emoji: ":marvin:"
        message: "[{{ module_name }} | {{ environment }} | {{ execution_id }} | {{ user }}] - {{ message }}"

In the previous install of st2, \n characters were honored and produced newlines in the output to slack. In the 2.7.2 release the \n characters get output, munging the output into a single line of output.

ISSUE TYPE

Pick one below and delete the rest:

• Bug Report

STACKSTORM VERSION

st2 2.7.2, on Python 2.7

OS / ENVIRONMENT / INSTALL METHOD

CentOS Linux release 7.5.1804 (Core)
Installation method:
curl -sSL https://stackstorm.com/packages/install.sh | bash -s -- --user=st2admin --password='REDACTED' --version="${st2_version}"

STEPS TO REPRODUCE

This outputs the same way ever time.

EXPECTED RESULTS

Expected output:

     Instance Id        State       IP Addr          Started         Avail Zone      Type       Environment        Role                 Full Name           
--------------------- ---------- ------------- -------------------- ------------ ------------ --------------- --------------- ------------------------------
i-0ab70a085095ff8f4   running    10.50.13.191  2018-08-08 15:15:54  us-west-2c   m5.large     staging02       biexport-worker biexport-worker-green_staging02
i-0c041493c86994868   running    10.50.12.76   2018-08-08 15:15:39  us-west-2b   m4.xlarge    staging02       cmm_web         cmm-green_staging02           
i-099b501ee05e2eb50   running    10.50.12.8    2018-05-01 03:22:48  us-west-2b   t2.medium    staging02       metabase        metabase-staging02            

ACTUAL RESULTS

     Instance Id        State       IP Addr          Started         Avail Zone      Type       Environment        Role                 Full Name           \n--------------------- ---------- ------------- -------------------- ------------ ------------ --------------- --------------- ------------------------------\ni-0ab70a085095ff8f4   running    10.50.13.191  2018-08-08 15:15:54  us-west-2c   m5.large     staging02       biexport-worker biexport-worker-green_staging02\ni-0c041493c86994868   running    10.50.12.76   2018-08-08 15:15:39  us-west-2b   m4.xlarge    staging02       cmm_web         cmm-green_staging02           \ni-099b501ee05e2eb50   running    10.50.12.8    2018-05-01 03:22:48  us-west-2b   t2.medium    staging02       metabase        metabase-staging02            \n ... 

Start with the workflow examples in workflows/list_instances.yaml and chains/post_message.yaml, then trace the slack.post_message entry point used by the chain. Reproduce the behavior with the supplied multiline instance list and verify that escaped \n sequences are rendered as line breaks in Slack output rather than literal text.