update mono in docker images

tools/dockerfile/distribtest/csharp_centos7_x64/Dockerfile

@@ -14,21 +14,11 @@
 
 FROM centos:7
 
-RUN rpm --import "http://keyserver.ubuntu.com/pks/lookup?op=get&search=0x3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF"
-RUN yum-config-manager --add-repo http://download.mono-project.com/repo/centos/
+RUN rpm --import "https://keyserver.ubuntu.com/pks/lookup?op=get&search=0x3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF"
+RUN curl https://download.mono-project.com/repo/centos7-stable.repo | tee /etc/yum.repos.d/mono-centos7-stable.repo
 
-RUN yum install -y mono
+RUN yum install -y mono-devel
 
-RUN yum install -y unzip
-
-# --nogpgcheck because nuget-2.12 package is not signed.
-RUN yum install -y nuget --nogpgcheck
+RUN yum install -y nuget
 
-# Help mono correctly locate libMonoPosixHelper.so
-# as a workaround for issue https://bugzilla.xamarin.com/show_bug.cgi?id=42820
-# The error message you'll get without this workaround:
-# ```
-# WARNING: /usr/lib/libMonoPosixHelper.so
-# WARNING: Unable to read package from path 'Grpc.1.1.0-dev.nupkg'.
-# ```
-RUN cp /usr/lib64/libMonoPosixHelper.so /usr/lib/
+RUN yum install -y unzip

tools/dockerfile/distribtest/csharp_ubuntu1404_x64/Dockerfile

@@ -14,15 +14,18 @@
 
 FROM ubuntu:14.04
 
-RUN apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF
-RUN echo "deb http://download.mono-project.com/repo/debian wheezy main" | tee /etc/apt/sources.list.d/mono-xamarin.list
+RUN apt-get update && apt-get install -y apt-transport-https && apt-get clean
+
+RUN apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF
+RUN echo "deb https://download.mono-project.com/repo/ubuntu stable-trusty main" | tee /etc/apt/sources.list.d/mono-official-stable.list
 
 RUN apt-get update && apt-get install -y \
     mono-devel \
     ca-certificates-mono \
-    nuget
+    nuget \
+    && apt-get clean
 
-RUN apt-get update && apt-get install -y unzip
+RUN apt-get update && apt-get install -y unzip && apt-get clean
 
 # Install dotnet CLI
 RUN apt-get install -y apt-transport-https

tools/dockerfile/distribtest/csharp_ubuntu1604_x64/Dockerfile

@@ -14,19 +14,15 @@
 
 FROM ubuntu:16.04
 
+RUN apt-get update && apt-get install -y apt-transport-https && apt-get clean
+
+RUN apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF
+RUN echo "deb https://download.mono-project.com/repo/ubuntu stable-xenial main" | tee /etc/apt/sources.list.d/mono-official-stable.list
+
 RUN apt-get update && apt-get install -y \
     mono-devel \
     ca-certificates-mono \
     nuget \
     && apt-get clean
 
-# make sure we have nuget 2.12+ (in case there's an older cached docker image)
-RUN apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF
-RUN echo "deb http://download.mono-project.com/repo/debian wheezy main" | tee /etc/apt/sources.list.d/mono-xamarin.list
-RUN apt-get update && apt-get install -y nuget && apt-get clean
-
-# Prevent "Error: SendFailure (Error writing headers)" when fetching nuget packages
-# See https://github.com/tianon/docker-brew-ubuntu-core/issues/86
-RUN apt-get update && apt-get install -y tzdata && apt-get clean
-
 RUN apt-get update && apt-get install -y unzip && apt-get clean