The zk-ca.sh tool for generating certificates fails with
and error that ./tools/openssl.cnf cannot be opened if a
relative path is supplied on ubuntu-bionic. So expand the
path to an absolute path instead.
This adds a script to generate TLS certs for zookeeper.
It also adds new config file options for specifying certs for a
TLS connection, adds a howto document to advise admins on how
to configure ZK for TLS.
It also removes the 'required' flag for the SASL auth parameters,
since they are not actually required.
Include the default openssl.cnf file since some distros modify it
to specify paths that are incompatbile with the zk-ca.sh script.