Refactored validation functionality and added rules

[JayjeetAtGithub]

Addresses #609

[pep8speaks]

Hello @JayjeetAtGithub! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

There are currently no PEP 8 issues detected in this Pull Request. Cheers! 🍻

Comment last updated at 2019-05-04 04:21:13 UTC

[JayjeetAtGithub]

@ivotron You may have a look

[ivotron]

thanks a lot @JayjeetAtGithub ! This is looking great! Some comments:

• We can rename Workflow to WorkflowRunner. And then create a Workflow class in the parser.py module. In this way, we separate the logic of creating the workflow and executing it. Workflow would represent the contents of the workflow; WorkflowRunner is in charge of executing it.
• The Workflow abstraction can encapsulate the workflow and be the interface to obtaining information about it (as opposed to representing the workflow via a dictionary as we do right now). This class can expose properties/methods such as name(), stages() (an iterator to iterate the stages of a workflow), etc..
• The complete_graph() function can also be part of the Workflow class.
• Since the check_secrets function is related to the execution of the workflow, we could leave it in this new WorkflowRunner class.

I know that, strictly speaking, the above is kind of out of the scope of this PR, so feel free to create another issue if you think we should address that separately. I figured we could take this opportunity to do this type of refactoring.

[JayjeetAtGithub]

Thanks a lot @ivotron ! I will implement the required changes shortly.

[JayjeetAtGithub]

@ivotron Please have a look !

[ivotron]

thanks a lot @JayjeetAtGithub !