[SPARK-14342][CORE][DOCS][TESTS] Remove straggler references to Tachyon

## What changes were proposed in this pull request?

Straggler references to Tachyon were removed:
- for docs, `tachyon` has been generalized as `off-heap memory`;
- for Mesos test suits, the key-value `tachyon:true`/`tachyon:false` has been changed to `os:centos`/`os:ubuntu`, since `os` is an example constrain used by the [Mesos official docs](http://mesos.apache.org/documentation/attributes-resources/).

## How was this patch tested?

Existing test suites.

Author: Liwei Lin <lwlin7@gmail.com>

Closes #12129 from lw-lin/tachyon-cleanup.

1 files changed, 2 insertions, 2 deletions

diff --git a/core/src/main/scala/org/apache/spark/scheduler/cluster/mesos/MesosSchedulerUtils.scala b/core/src/main/scala/org/apache/spark/scheduler/cluster/mesos/MesosSchedulerUtils.scala
index 35f914355d..233bdc23e6 100644
--- a/core/src/main/scala/org/apache/spark/scheduler/cluster/mesos/MesosSchedulerUtils.scala
+++ b/core/src/main/scala/org/apache/spark/scheduler/cluster/mesos/MesosSchedulerUtils.scala
@@ -283,11 +283,11 @@ private[mesos] trait MesosSchedulerUtils extends Logging {
    *  are separated by ':'. The ':' implies equality (for singular values) and "is one of" for
    *  multiple values (comma separated). For example:
    *  {{{
-   *  parseConstraintString("tachyon:true;zone:us-east-1a,us-east-1b")
+   *  parseConstraintString("os:centos7;zone:us-east-1a,us-east-1b")
    *  // would result in
    *  <code>
    *  Map(
-   *    "tachyon" -> Set("true"),
+   *    "os" -> Set("centos7"),
    *    "zone":   -> Set("us-east-1a", "us-east-1b")
    *  )
    *  }}}