Add performance improvement for Enum w/ Python 2 enum library

Add performance improvement for Enum w/ Python 2 enum library

Adjusted the initialization for :class:`.Enum` to minimize how often it
invokes the ``.__members__`` attribute of a given PEP-435 enumeration
object, to suit the case where this attribute is expensive to invoke, as is
the case for some popular third party enumeration libraries.

Fixes: #4758
Change-Id: Iffeb854c67393bdcb288944fc357a074e20e1325