cosa/cli-spec: add envVar CLI support

cosa/20200804-cfg-spec.md

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+# CoreOS Assembler Config Specification
+- Subject ID: COSA.20200804
+- Superceeds: None
+- Conflicts: None
+
+## Summary
+
+The RHCOS development pipelines use a "jobspec" to describe the pipeline runs.  This enhancement proposes a YAML-based interface based loosely on the RHCOS jobspec.
+
+## Goals
+
+1. Provide a file-based interface into COSA
+1. Prep for breaking up the Pipeline monoliths
+1. Provide a migration path for RHCOS pipelines
+1. Prepare for pipeline Nirvana (where *COS pipelines can live in peace and harmony)
+
+## Why?
+
+The innovation of the RHCOS JobSpec was it provided a clear seperation of configuration from the code; the logic was conflated. Further the jobspec allowed the logic to be run with different inputs.
+
+To support this, a "COSAspec" is being introduced to provide a CLI file-interface.
+
+## YAML to EnvVars
+
+* For Bash and Pythonic CLIs, each argument will have an envVar added.
+* At entry into COSA, the optional COSAspec will be checked
+* Values will be parsed and emit as EnvVars by a common entry point.
+* The EnvVars will set defaults to the CLI commands
+
+Note: The Mantle code is GoLang based. The use of envVars is supported by the Cobra CLI, but needs further research and consideration. The Mantle code is not considered in this design standard.
+
+## Types
+
+* List values will be joined to comma seperated strings
+* Dicts/Map will be flattened with an `_` between levels
+* Boolean values should be 0, 1, True, False, true, false.
+* Number types are strings and the consuming code is repsonsbile for casting to the type.
+
+Due to the lack of cohesion, the mix of Python, Bash and GoLang, each component will necessarily handle empty and null values.
+
+
+## Example:
+
+For the `coreos-assembler build` command:
+```
+build:
+  force: false
+  force_nocache: false
+  skip_prune: false
+  parent: <string>
+  tag: <string>
+```
+
+Would render the envVars:
+```
+COSA_BUILD_FORCE=false
+COSA_BUILD_FORCE_NOCAHCE=false
+COSA_BUILD_SKIP_PRUNE=false
+...
+```
+
+Then in `cmd-build` the defaults would be:
+```
+FORCE="${COSA_BUILD_FORCE:-}"
+FORCE_IMAGE="${COSA_BUILD_FORCE_IMAGE:-}"
+SKIP_PRUNE="${COSA_BUILD_SKIP_PRUNE:-0}"
+...
+```
+
+## List value example
+
+```
+extend:
+  aws:
+    regions:
+       - us-east-1
+       - us-west-1
+```
+Would render at `COSA_EXTEND_AWS_REGIONS="us-east-1,us-west-1"`
+
+## Dict example
+
+```
+foo:
+  bar:
+    baz:
+      1: true
+      2: true
+```
+
+Would render as `COSA_FOO_BAR_BAZ_1=true` and `COSA_FOO_BAR_BAZ_2=true`
+
+## PR Standard
+
+A thorough review of the COSA code-base, a comparison between the FCOS and RHCOS pipelines shows that each command will need to be handled seperately.
+
+Therefore, for each command updated to modify this interface:
+- The prefix `COSA_<COSA COMMAND>` will be used for envVars (e.g. `COSA_EXTEND_AZURE`)
+- No calculated values. Commands may not parse the spec file itself.
+- One command per PR.
+
+
+Naming convention for commands (in order of precedence):
+- `cmd-buildextend-*`: `COSA_EXTEND_*`
+- `cmd-build`: `COSA_BUILD_*`
+- `cmd-.*`: `COSA_<SUFFIX>_*`
+- `.*`: no interface