Taxii2 config docs and defaults

[andrewbeard]

There are a couple fields used for the taxii2 implementation that aren't described in the configurations docs, making setup a little harder than it should be. There are also a couple fields that are required (otherwise the server throws a 500 error) that aren't described as such.

While I was at it I figured rather than making setting a value for public_discovery required I'd just set it to a default of False. Right now discovery doesn't work at all (throws an error) unless the user explicitly sets an option, so False seemed like a safe default.