'Retrieve the storage group by name.
Check the storage group instance cache first to save
CLI call.
If the specified storage group doesn\'t exist in the cache,
try to grab it from CLI.
:param name: name of the storage group
:return: storage group instance'

def get_storage_group(self, name):

    if (name not in self.sg_cache):
        self.sg_cache[name] = self.vnx.get_sg(name)
    return self.sg_cache[name]
