nsqd: add --tls-min-version config option

This adds a new tls-min-version flag that enables nsqd users
to specify a min tls version acceptable. The arguments to this
flag, <v>, follow the format of Version<v> in the tls package.

This commit adds --tls-min-version=ssl30 to the test.sh flags.

This PR also go fmt's nsq_to_nsq, which has been unformatted for a while, and reduces code clutter from tlsRequiredOption.String.

closes #512.