cosa/cli-spec: add envVar CLI support

cosa/cli-spec.md

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+# CoreOS Assemlber 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 file-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 it the jobspec allowed the same logic to be run with different inputs.
+
+To support this, a "CliSpec" is being introduced.
+
+## YAML to EnvVars
+
+* For Bash Pythonic CLIs, each argument will have an envVar added.
+* At entry into COSA, the optional CliSpec will be checked
+* Values will be parsed and emit as EnvVars by a common entry point.
+
+## 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}"
+...
+```
+
+## 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_BUILDEXTEND_AZURE`)
+- No calculated values. Commands may not parse the spec file itself.
+- One command per PR.