Dropping support for builds on Centos 7

[Istvan Toth]

Hi,

As reported in YARN-11794 , building Hadoop on Centos 7 has been broken for
some time, and I don't see an easy way to fix it.

I propose removing Centos 7 from the supported build platforms, and
removing the CentOs 7 precommit tests from Jenkins HADOOP-19489 .

Note that this is a build isse, this does not affect running Hadoop
on CentOS.

Istvan