Ability to provide kerberos username and password to KerberosRestTemplate

[adeelmahmood]

Currently KerberosRestTemplate only works with keytab and service principal based credentials. It would be great if we can provide kerberos username and password at runtime to this class and be able to make rest calls using those credentials. That will be similar to a form based spnego authentication. An example use case is to access to webhdfs resources using KerberosRestTemplate.