now raises KeyError if connection name not found
now raises KeyError if connection name not found

This commit will cause client.config_for() method to now raise a KeyError
if the required config name is not found in the loaded configuration (or
if no configuration exists)

mzbenami/pyeapi,devtodor/pyeapi-py3,arista-eosplus/pyeapi,mitchmahan/pyeapi,jerearista/pyeapi,SivagnanamCiena/pyeapi