Set REQUIRED as Thrift Flag based default SSLPolicy
Set REQUIRED as Thrift Flag based default SSLPolicy

Summary: Change the default `SSLPolicy` defined through a Thrift Flag to `REQUIRED`.

Reviewed By: AjanthanAsogamoorthy

Differential Revision: D28933775

fbshipit-source-id: 8c06539c038dd6fbf34070125b10e1a2c02db151

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