Lock template steps

[jithine]

What happened:

As a template owner I want to prevent users from overriding my step. This is useful to provide the bare minimum steps a user must run to use my template. Also with Template composition involved I would like to prevent another Template from redefining a step which must run, let's say a step for doing security scanning.

What you expected to happen:

1. Provide a mechanism for template owner to specify that this step should not be overridden

eg:

steps:
 - run: ./run.sh
 - scan:
     command: ./scan.sh
     locked: true

2. User's shouldn't be able to override step scan in their screwdriver.yaml

How to reproduce it:

N/A

Related

There are few issues related to expanding a step's capabilities.

1. Continue step on failure.
2. Retry step on failure.

[jithine]

cc @klu909 @tkyi

[jithine]

How locked step interfaces with template composition.

Locked status of a step should be preserved in template composition.

1. If order is being used, a child template author must include a locked step, else it will throw an error.
   1. It will be up to child template author to figure out if position of the step in the order is correct.
2. A child template author cannot redefine a locked step.

While this results in a child template author to lose flexibility when dealing with steps of parent template, making a locked step immutable preserves the intent that this is a step which must run always and should run as it was originally defined.

[jithine]

Also one aspect to keep in mind is how to handle transition if a template owner makes a step locked which a user was overriding in the past.

[tkyi]

Working with these versions:
screwdrivercd/screwdriver - v4.1.65
screwdrivercd/ui - v1.0.602