[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/docs/running-on-mesos.md b/docs/running-on-mesos.md
index 293a82882e..8e47301a75 100644
--- a/docs/running-on-mesos.md
+++ b/docs/running-on-mesos.md
@@ -215,10 +215,10 @@ conf.set("spark.mesos.coarse", "false")
 You may also make use of `spark.mesos.constraints` to set attribute based constraints on mesos resource offers. By default, all resource offers will be accepted.
 
 {% highlight scala %}
-conf.set("spark.mesos.constraints", "tachyon:true;us-east-1:false")
+conf.set("spark.mesos.constraints", "os:centos7;us-east-1:false")
 {% endhighlight %}
 
-For example, Let's say `spark.mesos.constraints` is set to `tachyon:true;us-east-1:false`, then the resource offers will be checked to see if they meet both these constraints and only then will be accepted to start new executors.
+For example, Let's say `spark.mesos.constraints` is set to `os:centos7;us-east-1:false`, then the resource offers will be checked to see if they meet both these constraints and only then will be accepted to start new executors.
 
 # Mesos Docker Support