[Spark-8308] [MLlib] add missing save load for python example

[hhbyyh]

jira: https://issues.apache.org/jira/browse/SPARK-8308

1. add some missing save/load in python examples. , LogisticRegression, LinearRegression and NaiveBayes
2. tune down iterations for MatrixFactorization, since current number will trigger StackOverflow for default java configuration (>1M)

[SparkQA]

Test build #34686 has finished for PR 6760 at commit 3e948dc.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[jkbradley]

@hhbyyh Can you please make the Scala example use 20 iterations too? Other than that, it looks good.

[SparkQA]

Test build #36082 has finished for PR 6760 at commit 9bd3383.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[hhbyyh]

@jkbradley Glad you're back! Just sent an update. Thanks for reviewing.

[jkbradley]

LGTM merging into master
Thank you!