The documentation on Kerberos should mention that the Linux packages we publish can be configured by dropping a setting into a file the init script/upstart script picks up.

On RedHat/Fedora/Centos, the file is at /etc/sysconfig/mongod. On Debian/Ubuntu the file is at /etc/default/mongodb. The magic line is

export KRB5_KTNAME="xxxxxx"

For suitable values of xxxxxx.

The relevant doc page is here:

http://docs.mongodb.org/manual/tutorial/control-access-to-mongodb-with-kerberos-authentication/