osclib/cache: provide mechanism to init multiple times.

Useful for staging plugin to cache the remote config before StagingAPI
is initialized.

osc-staging.py

@@ -389,6 +389,9 @@ def do_staging(self, subcmd, opts, *args):
         else:
             opts.project = 'Factory'
 
+    # Cache the remote config fetch.
+    Cache.init()
+
     # Init the OBS access and configuration
     opts.project = self._full_project_name(opts.project)
     opts.apiurl = self.get_api_url()

osclib/cache.py

@@ -103,6 +103,11 @@ class Cache(object):
 
     @staticmethod
     def init(directory='main'):
+        if Cache.CACHE_DIR:
+            # Stick with the first initialization to allow for StagingAPI to
+            # ensure always enabled, but allow parent to change directory.
+            return
+
         Cache.CACHE_DIR = CacheManager.directory('request', directory)
 
         Cache.patterns = []