Support single argument version of sqlContext.getConf

## What changes were proposed in this pull request?

In Python, sqlContext.getConf didn't allow getting the system default (getConf with one parameter).

Now the following are supported:
```
sqlContext.getConf(confName)  # System default if not locally set, this is new
sqlContext.getConf(confName, myDefault)  # myDefault if not locally set, old behavior
```

I also added doctests to this function. The original behavior does not change.

## How was this patch tested?

Manually, but doctests were added.

Author: mathieu longtin <mathieu.longtin@nuance.com>

Closes #12488 from mathieulongtin/pyfixgetconf3.