• Refactor retries in the Mistral action runner to use exponential backoff. Configuration options
  for Mistral have changed. (improvement)
• Add SSH bastion host support to the paramiko SSH runner. Utilizes same connection parameters as
  the targeted box. (new feature, improvement) #2144, #2150 [Logan Attwood]
• Update action chain runner so it performs on-success and on-error task name validation during
  pre_run time. This way common errors such as typos in the task names can be spotted early on
  since there is no need to wait for the run time.
• Change headers and params core.http action paramer type from string to
  object.
• Don't allow action parameter type attribute to be an array since rest of the code doesn't
  support parameters with multiple types. (improvement)
• Fix trigger parameters validation for system triggers during rule creation - make sure we
  validate the parameters before creating a TriggerDB object. (bug fix)
• Update local runner so all the commands which are executed as a different user and result in
  using sudo set $HOME variable to the home directory of the target user. (improvement)
• Fix a bug with a user inside the context of the live action which was created using alias
  execution endpoint incorrectly being set to the system user (stanley) instead of the
  authenticated user which triggered the execution. (bug fix)
• Include state_info for Mistral workflow and tasks in the action execution result. (improvement)
• Introduce a new timeout action execution status which represents an action execution
  timeout. Previously, executions which timed out had status set to failure. Keep in mind
  that timeout is just a special type of a failure. (new feature)
• --debug flag no longer implies profiling mode. If you want to enable profiling mode, you need
  to explicitly pass --profile flag to the binary. To reproduce the old behavior, simply pass
  both flags to the binary - --debug --profile.
• Fix policy loading and registering - make sure we validate policy parameters against the
  parameters schema when loading / registering policies. (bug fix, improvement)
• Fix policy trigger for action execution cancellation. (bug fix)
• Improve error reporting for static error in ActionChain definition e.g. incorrect reference
  in default etc. (improvement)
• Fix action chain so it doesn't end up in an infinite loop if an action which is part of the chain
  is canceled. (bug fix)
• Allow jinja templating to be used in message and data field for notifications.(new feature)
• Add tools for purging executions (also, liveactions with it) and trigger instances older than
  certain UTC timestamp from the db in bulk.
• Fix json representation of trace in cli. (bug fix)
• Introducing noop runner and core.noop action. Returns consistent success in a WF regardless of
  user input. (new feature)
• Add missing indexes on trigger_instance_d_b collection. (bug fix)
• Add mock classes (st2tests.mocks.*) for easier unit testing of the packs. (new feature)
• Add a script (./st2common/bin/st2-run-pack-tests) for running pack tests. (new feature)
• Modify ActionAliasFormatParser to work with regular expressions and support more flexible parameter matching. (improvement)
• Move ChatOps pack to st2 core.
• Support for formatting of alias acknowledgement and result messages in AliasExecution. (new feature)
• Support for "representation+value" format strings in aliases. (new feature)
• Support for disabled result and acknowledgement messages in aliases. (new feature)
• Add ability to write rule enforcement (models that represent a rule evaluation that resulted
  in an action execution) to db to help debugging rules easier. Also, CLI bindings to list and view these models are added. (new-feature)