Enable building dotnet-CLI for centos.

I had to patch up a redhat dnx package which supports NETStandard.Library package. It is currently uploaded to dotnet-cli blob storage. This hack will no longer be required when we move to xplat nuget to do 'dotnet restore'. Apart from this there are three issues that are tracked for centos.
- compile-native not yet supported - https://github.com/dotnet/cli/issues/453
- dnu restore crashes intermittently on centos. I need to investigate this a little bit more and file issues on dnx or coreclr. This will make our CI builds very flaky.
- Dotnet restore does not restore native shims when using “centos.7-x64” - https://github.com/dotnet/corefx/issues/5066

netci.groovy

@@ -7,11 +7,12 @@ import jobs.generation.Utilities;
 
 def project = GithubProject
 
-def osList = ['Ubuntu', 'OSX', 'Windows_NT']
+def osList = ['Ubuntu', 'OSX', 'Windows_NT', 'CentOS7.1']
 
 def machineLabelMap = ['Ubuntu':'ubuntu-doc',
                        'OSX':'mac',
-                       'Windows_NT':'windows']
+                       'Windows_NT':'windows',
+                       'CentOS7.1' : 'centos-71']
 
 def static getBuildJobName(def configuration, def os) {
     return configuration.toLowerCase() + '_' + os.toLowerCase()