Adding support to create cluster for tinc testing. This also create
gptest database. Here is an example script to run tinc tests:

```
cd ~/workspace/gpdb
source .../greenplum_path.sh
make create-tinc-test-cluster
source gpAux/gpdemo/gpdemo-env.sh
make -C src/test/tinc walrep_2 # to run walrep_2 tinc tests
```

Author: Xin Zhang <xzhang@pivotal.io>
Author: Ashwin Agrawal <aagrawal@pivotal.io>