[REEF-1584] Fix CoreCLR Issues in Org.Apache.REEF.IO

This addressed the issue by
  * Targeting .NET Core 2.0 Framework
  * Rewriting the DirectoryInfo.Create(DirectorySecurity) and CreateSubdirectory(DirectorySecurity) commands to use the SetAccessControl APIs that are supported in .NET Core 2.0
    * See the Extension Methods section of the [.NET Standard 2.0 documentation](https://docs.microsoft.com/en-us/dotnet/api/system.io.directoryinfo?view=netstandard-2.0) for details on the supported APIs. (Note that these extensions are all .NET Core 2.0, not .NET Standard.)

JIRA:
  [REEF-1584](https://issues.apache.org/jira/browse/REEF-1584)

Pull request:
  This closes #1351
diff --git a/lang/cs/Org.Apache.REEF.IO/Files/DefaultDirectoryInfo.cs b/lang/cs/Org.Apache.REEF.IO/Files/DefaultDirectoryInfo.cs
index 47a86fe..29011a8 100644
--- a/lang/cs/Org.Apache.REEF.IO/Files/DefaultDirectoryInfo.cs
+++ b/lang/cs/Org.Apache.REEF.IO/Files/DefaultDirectoryInfo.cs
@@ -180,7 +180,8 @@
         /// </summary>
         public void Create(DirectorySecurity directorySecurity)
         {
-            _directoryInfo.Create(directorySecurity);
+            _directoryInfo.Create();
+            _directoryInfo.SetAccessControl(directorySecurity);
         }
 
         /// <summary>
@@ -196,7 +197,9 @@
         /// </summary>
         public IDirectoryInfo CreateSubdirectory(string path, DirectorySecurity directorySecurity)
         {
-            return FromDirectoryInfo(_directoryInfo.CreateSubdirectory(path, directorySecurity));
+            var directoryInfo = CreateSubdirectory(path);
+            directoryInfo.SetAccessControl(directorySecurity);
+            return directoryInfo;
         }
 
         /// <summary>