Merge ssl-version-default-7684: Disable SSLv3 by default for string server endpoints.

Author: Alex

Reviewer: glyph

Fixes: #7684

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git-svn-id: svn://svn.twistedmatrix.com/svn/Twisted/trunk@43395 bbbe8e31-12d6-0310-92fd-ac37d47ddeeb