Add RedHat6 to official builds of core-setup in master branch (#3024)

* Add RedHat6 to official builds of core-setup in master branch

Add RedHat6 to official builds of core-setup in master branch, this is
identical to the PR to enable Rhel6 in release/2.0.0:
https://github.com/dotnet/core-setup/pull/3017

* Rename the RID.

Rename the RID as we don't have a RID for 6.9.

* Update docker tag and add logic to detect RHEL6 in init-tools.sh

Update docker tag and add logic to detect RHEL6 in init-tools.sh

* Port changes from Release/2.0.0 to fix core-setup RHEL 6 official runs.

Port changes from Release/2.0.0 to fix core-setup RHEL 6 official runs.

* Fix indentation

Fix indentation

* Port one more related commit from release/2.0.0 back to master

Port one more related commit from release/2.0.0 back to master, which
is:

15e34cd2c5

* Update to use the correction version of runtimemodel

Update to use the correction version of runtimemodel

* Try to fix the CI error by updating NugetModelTFM to net45

Try to fix the CI error by updating NugetModelTFM to net45

* Revert "Try to fix the CI error by updating NugetModelTFM to net45"

This reverts commit d740f18750362245427b67ca45301f60b31faa47.

* Remove runtimemodel reference

I built core-setup locally, and didn't see runtimemodel package any more
with current version 4.3.0-preview2-4095. It is possible that it might
not be needed any longer.

Remove it first and verify it all works. If there is still runtime
errors then we may need to explicitly add a package reference to it

* Remove unused NugetModelTFM node as well

Remove unused NugetModelTFM node as well

* Remove extra spaces

Remove extra spaces

buildpipeline/pipeline.json

@@ -30,6 +30,21 @@
             "Platform": "x64"
           }
         },
+        {
+          "Name": "Core-Setup-Linux-Arm-BT",
+          "Parameters": {
+            "PB_DistroRid": "rhel.6-x64",
+            "PB_DockerTag": "centos-6-c8c9b08-20174310104313",
+            "PB_TargetArchitecture": "x64",
+            "PB_AdditionalBuildArguments":"-TargetArchitecture=x64 -DistroRid=rhel.6-x64 -PortableBuild=false -strip-symbols",
+            "PB_PortableBuild": "false"
+          },
+          "ReportingParameters": {
+            "OperatingSystem": "RedHat6",
+            "Type": "build/product/",
+            "Platform": "x64"
+          }
+        },
         {
           "Name": "Core-Setup-Linux-Arm-BT",
           "Parameters": {

init-tools.sh

@@ -33,6 +33,14 @@ OSName=$(uname -s)
         Linux)
             __DOTNET_PKG=dotnet-dev-linux-x64
             OS=Linux
+			
+            if [ -e /etc/redhat-release ]; then
+                redhatRelease=$(</etc/redhat-release)
+                if [[ $redhatRelease == "CentOS release 6."* || $redhatRelease == "Red Hat Enterprise Linux Server release 6."* ]]; then
+                    __DOTNET_PKG=dotnet-dev-rhel.6-x64
+                fi
+            fi			
+			
             ;;
 
         *)