Set minimum TLS protocol version to TLSv1.2, set PreferServerCipherSuites

Some environments are subject to strict rules about the permitted TLS
protocol verion and available ciphers. Setting TLSv1.2 as the minimum
version ensures we do not use weaker protocols. We've opted against
making this configurable given the age of TLSv1.2 and the increasing
push to deprecate TLSv1.1 and older.

The PreferServerCipherSuites setting is also commonly flagged by SSL
quality scanning tools. Since Go provides a relatively modern set of
default ciphers by default, defaulting this to true is unlikely to
make much practical difference.

Signed-off-by: Steven Danna <steve@chef.io>