[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.