This commit makes modifications to move backend
code to pluggy plugins which will exist in other
repos.  The exception to this is the None backend
type which remains present.  Modifications in
this initial restructuring include:

- Deletion of files to be moved to other repos

- Addition of files related to pluggy hooks
  - Ex: nixops/plugins/* files

- Modification of files to utilize pluggy hooks
  and pluggy resources
  - Ex: eval-machine-info.nix

- release.nix changes to enable using nixops
  with plugins using CLI patterns of:

nix-build release.nix -A build.x86_64-linux --arg p "(p: [ p.plugin1 ])"

./dev-shell --arg p "(p: [ p.plugin1 p.plugin2 ])"

./dev-shell --arg p "(p: [ (p.callPackage ../myplugin/release.nix {}) ])"

- Modification of scripts/nixops to include a
  new command:

nixops list-plugins [-v]

To enable plugin CLI extensibility, this commit extracts
the function definitions out of scripts/nixops and places
them in nixops/script_defs.py.  Explicit argument passing
is now done where global variables were initially utilized
in the script.  This way, script functions are now
available to plugins for extensibility purposes.

This commit makes modifications to move backend
code to pluggy plugins which will exist in other
repos.  The exception to this is the None backend
type which remains present.  Modifications in
this initial restructuring include:

- Deletion of files to be moved to other repos

- Addition of files related to pluggy hooks
  - Ex: nixops/plugins/* files

- Modification of files to utilize pluggy hooks
  and pluggy resources
  - Ex: eval-machine-info.nix

- release.nix changes to enable using nixops
  with plugins using CLI patterns of:

nix-build release.nix -A build.x86_64-linux --arg p "(p: [ p.plugin1 ])"

./dev-shell --arg p "(p: [ p.plugin1 p.plugin2 ])"

./dev-shell --arg p "(p: [ (p.callPackage ../myplugin/release.nix {}) ])"

- Modification of scripts/nixops to include a
  new command:

nixops list-plugins [-v]

To enable plugin CLI extensibility, this commit extracts
the function definitions out of scripts/nixops and places
them in nixops/script_defs.py.  Explicit argument passing
is now done where global variables were initially utilized
in the script.  This way, script functions are now
available to plugins for extensibility purposes.

also make the exported nix expressions from the plugins part of the
search path which useful for backends such as VirtualBox.

nixops-core-pr update

* To be migrated to the virtualbox plugin

Nixops Core, Plugin Restructuring PR