Add basic workflow generation

[cfournie]

This PR adds to the WorkflowApp class the ability to serialize to XML action and kill nodes using the oozie-workflow-0.5.xsd schema.

Implementation

This PR implements:

• An abstract _WorkflowEntity class to serve as the base type for these and other structures that will be exposed to users to allow them to define a workflow which features:
  • An optional on_error parameter which allows one to specify a _WorkflowEntity to transition to if an error occurs
  • An optional name to help document identifiers in the XML (otherwise a uid is used)
• A Kill _WorkflowEntity which if transitioned to kills a workflow and emits a message
• An Action _WorkflowEntity which wraps a ConcreteAction (e.g. execute a shell command, subworkflow, or email) which
  • Can specify credentials that are to be used by a concrete action
  • Can specify action retry semantics

This PR also adjusts WorkflowApp to validate that:

• All credentials referred to by actions are supplied in the constructor
• No duplicate identifiers are present in the actions provided (an Oozie logic error)

This PR enables the ability to define collections of actions that can be executed serially/in parallel (https://github.com/Shopify/pyoozie/pull/50).

🎩 'd by starting a local Oozie 4.1.0 instance using docker-compose up -d and manipulating tests/pyoozie/test_interactive.py to specify different values.

Notes

This PR is one of several that are intended to add an API to define forking/joining workflows with error-handling prototyped in https://github.com/Shopify/pyoozie/pull/26.

[cfournie]

@honkfestival @prabcs @sabidib @solackerman any final comments?