Elytron subsystem attributes with a default value should not be configured as required

A number of elytron subsystem attributes are configured as being required even though they have a default value (which implies not requiring a user-provided setting).

Once WFCORE-2249 is fixed which results in proper validation of complex object attributes, this bug results in the server failing to boot:

2017-01-31 12:28:04,055 ERROR [org.jboss.as.controller.management-operation] (ServerService Thread Pool -- 13) WFLYCTL0013: Operation ("add") failed - address: ([
    ("subsystem" => "elytron"),
    ("mechanism-provider-filtering-sasl-server-factory" => "elytron")
]) - failure description: "WFLYCTL0155: 'version-comparison' may not be null"

Simple fix, just change the definition.

I found 13 attributes like this.