Pass extra args to custom create command (#972)

This is needed to allow custom deploy commands to handle arbitrary args.

* Adds a `DeploymentContext.modify_yaml` helper
* Removes `laconicd` from test stack to simplify it

Reviewed-on: cerc-io/stack-orchestrator#972
Reviewed-by: ashwin <ashwin@noreply.git.vdb.to>

stack_orchestrator/data/stacks/test/stack.yml

@@ -2,7 +2,6 @@ version: "1.0"
 name: test
 description: "A test stack"
 repos:
-  - git.vdb.to/cerc-io/laconicd
   - git.vdb.to/cerc-io/test-project@test-branch
 containers:
   - cerc/test-container