[Thrift] Make principal configurable in DemoClient.java

In the Thrift1 demo client we have this code:

transport = new TSaslClientTransport("GSSAPI", null,
  "hbase", // Thrift server user name, should be an authorized proxy user.
  host, // Thrift server domain
  saslProperties, null, transport);

This will only work when the Thrift server is started with the hbase principal. Often this may deviate, for example I am using hbase-thrift to separate the names from those of backend servers.

What we need is either an additional command line option to specify the name, or a property that can be set with -D and can be passed at runtime. I prefer the former, as the latter is making this a little convoluted.