Enable alternative authentication to Accumulo

[n3101]

Gaffer authenticates to Accumulo using stored username & password. There is now a requirement to offer a choice of authentication either using Kerberos or the existing username/password.

(https://github.com/gchq/Gaffer/issues/2348 previously enquired about setting up Gaffer to authenticate to Accumulo using Kerberos)

This needs to work with Accumulo 2. If possible also make it work for Accumulo 1.