Amend documentation on keytab file location to mention the possibility to use the environment variable KRB5_KTNAME

Documentation page:

https://github.com/mariadb-corporation/MaxScale/blob/2.5/Documentation/Authenticators/GSSAPI-Authenticator.md

Currently the page says: "The keytab file must be placed in the configured default location which almost always is /etc/krb5.keytab."

It has been tested that setting the environment variable KRB5_KTNAME works as expected and documented here:

https://web.mit.edu/kerberos/krb5-latest/doc/basic/keytab_def.html

I think it would be useful to mention that in the documentation, this allows users to know they can use custom paths for they keytab file.