Heads Up: new (test-only) system dependencies

[Dan Burkert <da...@apache.org>]

Hi all,

As you can see in the recent commit history, work has gotten started on
adding security features to Kudu.  As part of this, we are now relying on
MIT Kerberos (krb5), OpenSSL, and the GSSAPI SASL plugin.  Since these are
security-sensitive dependencies, we are relying on the system to provide
them (meaning they need to be pre-installed).

Currently the dependencies are only needed for tests, but eventually they
will be required for running Kudu, at least in secure mode.  At that point
installation.adoc and other user documentation will be updated accordingly.

OS X install instructions (Homebrew):
--------------------------------------------------
brew install homebrew/dupes/krb5

Ubuntu install instructions:
-------------------------------
apt-get install openssl
apt-get install krb5-admin-server krb5-user krb5-kdc
apt-get install libsasl2-modules-gssapi-mit

RHEL install instructions:
-----------------------------
yum install openssl
yum install krb5-workstation krb5-server
yum install cyrus-sasl-gssapi

- Dan