remove: dnf.yum.plugins and all references.

To be replaced by dnf.plugins.
cli: commands: state whether the command might try to modify rpmdb.

so a plugin can stop it quickly when the rpmdb is not writable for the current
user.
plugins: basic plugins loading and running.
plugins: test plugin loading
plugins: loading from the CLI.

Changes the default plugin and pluginconfig paths. Plugins are on by default.
remove: conf.keepcache option.

No real use.
doc: api: Base.reset()
doc: api: logging.