[SPARK-6258] [MLLIB] GaussianMixture Python API parity check

Implement Python API for major disparities of GaussianMixture cluster algorithm between Scala & Python
```scala
GaussianMixture
    setInitialModel
GaussianMixtureModel
    k
```

Author: Yanbo Liang <ybliang8@gmail.com>

Closes #6087 from yanboliang/spark-6258 and squashes the following commits:

b3af21c [Yanbo Liang] fix typo
2b645c1 [Yanbo Liang] fix doc
638b4b7 [Yanbo Liang] address comments
b5bcade [Yanbo Liang] GaussianMixture Python API parity check