pluginlib: Update metaplugin to use commctx.

pluginlib: Update metaplugin to use commctx.

This is a final metaplugin refactoring which allow to move
all metaplugin based plugins (kubernetes, swift and rhv) to use
a common code base removing all unwanted coding and introduce
a static configuration only. Now all you need to define a next
metaplugin is to define a following variables and link metaplugin.lo:

// Plugin Info definitions
extern const char *PLUGIN_LICENSE;
extern const char *PLUGIN_AUTHOR;
extern const char *PLUGIN_DATE;
extern const char *PLUGIN_VERSION;
extern const char *PLUGIN_DESCRIPTION;

// Plugin linking time variables
extern const char *PLUGINPREFIX;
extern const char *PLUGINNAME;
extern const char *PLUGINAPI;
extern const char *BACKEND_CMD;

// The list of restore options saved to the RestoreObject.
extern struct ini_items plugin_items_dump[];

// the list of valid plugin options
extern const char *valid_params[];