SPARK-3177 (on Master Branch)

The JIRA and PR was original created for branch-1.1, and move to master branch now.
Chester

The Issue is due to that yarn-alpha and yarn have different APIs for certain class fields. In this particular case,  the ClientBase using reflection to to address this issue, and we need to different way to test the ClientBase's method.  Original ClientBaseSuite using getFieldValue() method to do this. But it doesn't work for yarn-alpha as the API returns an array of String instead of just String (which is the case for Yarn-stable API).

 To fix the test, I add a new method

  def getFieldValue2[A: ClassTag, A1: ClassTag, B](clazz: Class[_], field: String,
                                                      defaults: => B)
                              (mapTo:  A => B)(mapTo1: A1 => B) : B =
    Try(clazz.getField(field)).map(_.get(null)).map {
      case v: A => mapTo(v)
      case v1: A1 => mapTo1(v1)
      case _ => defaults
    }.toOption.getOrElse(defaults)

to handle the cases where the field type can be either type A or A1. In this new method the type A or A1 is pattern matched and corresponding mapTo function (mapTo or mapTo1) is used.

Author: chesterxgchen <chester@alpinenow.com>

Closes #2204 from chesterxgchen/SPARK-3177-master and squashes the following commits:

e72a6ea [chesterxgchen]  The Issue is due to that yarn-alpha and yarn have different APIs for certain class fields. In this particular case,  the ClientBase using reflection to to address this issue, and we need to different way to test the ClientBase's method.  Original ClientBaseSuite using getFieldValue() method to do this. But it doesn't work for yarn-alpha as the API returns an array of String instead of just String (which is the case for Yarn-stable API).
[Docs] minor grammar fix

Author: Nicholas Chammas <nicholas.chammas@gmail.com>

Closes #2430 from nchammas/patch-2 and squashes the following commits:

d476bfb [Nicholas Chammas] [Docs] minor grammar fix
[SQL][DOCS] Improve table caching section

Author: Michael Armbrust <michael@databricks.com>

Closes #2434 from marmbrus/patch-1 and squashes the following commits:

67215be [Michael Armbrust] [SQL][DOCS] Improve table caching section
[SPARK-1455] [SPARK-3534] [Build] When possible, run SQL tests only.

If the only files changed are related to SQL, then only run the SQL tests.

This patch includes some cosmetic/maintainability refactoring. I would be more than happy to undo some of these changes if they are inappropriate.

We can accept this patch mostly as-is and address the immediate need documented in [SPARK-3534](https://issues.apache.org/jira/browse/SPARK-3534), or we can keep it open until a satisfactory solution along the lines [discussed here](https://issues.apache.org/jira/browse/SPARK-1455?focusedCommentId=14136424&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-14136424) is reached.

Note: I had to hack this patch up to test it locally, so what I'm submitting here and what I tested are technically different.

Author: Nicholas Chammas <nicholas.chammas@gmail.com>

Closes #2420 from nchammas/selective-testing and squashes the following commits:

db3fa2d [Nicholas Chammas] diff against master!
f9e23f6 [Nicholas Chammas] when possible, run SQL tests only