I did some research and apparently this has to do with the operating system not having enough 'entropy' to create 'quality randomness' for cryptography. Here is a definition I found helpful:
In computing, entropy is the randomness collected by an operating system or application for use in cryptography or other uses that require random data. This randomness is often collected from hardware sources, either pre-existing ones such as mouse movements or specially provided randomness generators.Apparently you can see how much 'entropy' your operating system has available by looking at the number contained in the file "/proc/sys/kernel/random/entropy_avail"
When the creation of the kerberos db was hanging, that file was reporting an entropy value of around 75.
So, I found this handy utility called 'haveged' (HArdware Volatile Entropy Gathering and Expansion), available here: http://www.issihosts.com/haveged/downloads.html
More info: http://www.irisa.fr/caps/projects/hipsor/
Of course I had to install a C++ compiler: "yum install -y gcc-c++.x86_64"
Then, $ tar -xvf haveged-x.x.tar.gz
Change into the haveged directory,
$ make install
$ haveged -w 1024 #(this starts the daemon)
$ echo "/usr/local/sbin/haveged -w 1024" >> /etc/rc.local #(this starts the daemon on boot)
Once this was done, my entropy available was showing values between 2000 and 3000. When I then attempted to create the kerberos DB, it finished within a second! No problems!
Looks like I also needed to clear out the mess of 'principal' files in /var/kerberos/krb5kdc/ before re running the create.