Add param for specifying validate connection script.

* Add param `validcon_path` in `postgresql::client` (defaults to
previous hard coded value).
* Add tests for this new parameter.

All tests runs successfully on Scientific Linux 6.4

```
$ bundle exec rake spec SPEC_OPTS='--format documentation'
[...]

Finished in 1 minute 58.61 seconds
159 examples, 0 failures
```
Rename parameter validcon_path to validcon_script_path.

* As requested by @apenney, rename parameter
* Test suite passed successfully
Remove documentation for parameter `validcon_path` from private class `postgresql::globals`.

* As requested by @igalic, as it was a duplicate of `validcon_path` in `postgresql::client`.