Change cipher default strings to a function

Making the default cipher strings a function gives the library more
control over the defaults. Potentially allowing a change in the
future as ciphers become deprecated or dangerous.
Also allows third party distributors to change the defaults for their
installations.

Reviewed-by: Paul Yang <yang.yang@baishancloud.com>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/8686)