ACCUMULO-3317 Restrict Jetty secure protocol to TLS by default.

Warn if the user configures SSL instead of the default TLS
Merge branch '1.5' into 1.6

Conflicts:
	core/src/main/java/org/apache/accumulo/core/conf/Property.java
	server/base/src/main/java/org/apache/accumulo/server/Accumulo.java
	server/src/main/java/org/apache/accumulo/server/util/EmbeddedWebServer.java