plugins: Refactor plugin identification and access

This change moves all plugin handling code into the kas.plugins module.
New accessor functions `plugins.get(name)` and `plugins.all()` are
provided to wrap the plugins dictionary so that the kas main function
doesn't need to worry about how this is accessed. Plugins are loaded at
runtime rather than at parse time by calling `plugins.load()` which
gives us an improved ability to handle errors.

The `@kasplugin` decorator is removed as it modified and attribute on
the kasplugin function itself when a plugin module was loaded. Importing
a module should not result in changes to a variable in a different
module as it leads to an initialization code flow which is difficult to
reason about. Instead, plugin modules should now list the plugins which
they introduce in a `__KAS_PLUGINS__` list which will be walked at
runtime by `plugins.load()`.

Signed-off-by: Paul Barker <pbarker@konsulko.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

kas/plugins/shell.py

@@ -35,13 +35,11 @@ from kas.libcmds import (Macro, Command, SetupDir, SetupEnviron,
                          CleanupSSHAgent, SetupSSHAgent,
                          Loop, InitSetupRepos, FinishSetupRepos,
                          SetupReposStep)
-from kas.plugins import kasplugin
 
 __license__ = 'MIT'
 __copyright__ = 'Copyright (c) Siemens AG, 2017-2018'
 
 
-@kasplugin
 class Shell:
     """
         Implements a kas plugin that opens a shell within the kas environment.
@@ -129,3 +127,6 @@ class ShellCommand(Command):
         if ret != 0:
             logging.error('Shell returned non-zero exit status %d', ret)
             sys.exit(ret)
+
+
+__KAS_PLUGINS__ = [Shell]