Zookeeper: Add variables for Quorum Authorization with SASL

[jeqo]

Hi, I'd like to propose the following variables to support Quorum Authorization based on SASL:

Ref: https://cwiki.apache.org/confluence/display/ZOOKEEPER/Server-Server+mutual+authentication

quorum.auth.enableSasl=true
quorum.auth.learnerRequireSasl=true
quorum.auth.serverRequireSasl=true
quorum.auth.learner.saslLoginContext=QuorumLearner
quorum.auth.server.saslLoginContext=QuorumServer
quorum.cnxn.threads.size=20

Haven't considered Kerberos service, but if needed I can add it as well.

[jrevillard]

I'm very much interested into this, can someone merge it ?