Allow to use the specific version of ptest

You can use a different version of ptests on a single OS with this patch. It works for me.

* build different versions of Pacemakers in different directories.
* make install only the first time (to install /usr/share/pacemaker/*.rng)
* Specify PTEST environment variable as the path for the ptest in the build directory which you want to run

ex.
```
[root@devnode1 Trac2356-ptest-diff]# PTEST=/root/ClusterLabs/pacemaker-1.0.12/pengine/ptest ./ptest-cutter.sh test-33
ptest version is 1.0.12
Making log file of pe-input-1464.bz2
Making log file of pe-input-1465.bz2
Cutting pe-input-1464.log
Cutting pe-input-1465.log
done

Let's compare "1.0.12/ptest-cut" directory to another version's directory

[root@devnode1 Trac2356-ptest-diff]# PTEST=/root/ClusterLabs/pacemaker-1.0.13/pengine/ptest ./ptest-cutter.sh test-33
ptest version is 1.0.13
Making log file of pe-input-1464.bz2
Making log file of pe-input-1465.bz2
Cutting pe-input-1464.log
Cutting pe-input-1465.log
done

Let's compare "1.0.13/ptest-cut" directory to another version's directory

[root@devnode1 Trac2356-ptest-diff]# diff 1.0.12/ptest-cut/ 1.0.13/ptest-cut/
diff 1.0.12/ptest-cut/pe-input-1464.log 1.0.13/ptest-cut/pe-input-1464.log
29,36d28
< notice: RecurringOp:  Start recurring monitor (10s) for vip-app on ibm02
< notice: RecurringOp:  Start recurring monitor (10s) for vip-master on ibm02
(...omit...)
```