Don't load the grains again when printing them via salt-call -g

The BaseCaller instance already has an SMinion stored in self.minion,
and that SMinion instance compiles grains in its dunder init. This
commit uses the SMinion's grains if it can find them, and only compiles
fresh grains as a last resort. Even that seems to be erring on the side
of caution, since the SMinion should always have grains.

This prevents salt-call from compiling grains twice in a single run,
resulting in a 40-50% speed reduction when running `salt-call -g`.

salt/cli/caller.py

@@ -120,7 +120,7 @@ class BaseCaller(object):
         '''
         Print out the grains
         '''
-        grains = salt.loader.grains(self.opts)
+        grains = self.minion.opts.get('grains') or salt.loader.grains(self.opts)
         salt.output.display_output({'local': grains}, 'grains', self.opts)
 
     def run(self):