SPARK-1890 and SPARK-1891- add admin and modify acls

It was easier to combine these 2 jira since they touch many of the same places.  This pr adds the following:

- adds modify acls
- adds admin acls (list of admins/users that get added to both view and modify acls)
- modify Kill button on UI to take modify acls into account
- changes config name of spark.ui.acls.enable to spark.acls.enable since I choose poorly in original name. We keep backwards compatibility so people can still use spark.ui.acls.enable. The acls should apply to any web ui as well as any CLI interfaces.
- send view and modify acls information on to YARN so that YARN interfaces can use (yarn cli for killing applications for example).
More out of curiosity: since the class is already "private[spark]", is the modifier also needed here?