get the right nuget version in linux distribtests

tools/dockerfile/distribtest/csharp_centos7_x64/Dockerfile

@@ -34,6 +34,6 @@ RUN yum-config-manager --add-repo http://download.mono-project.com/repo/centos/
 
 RUN yum install -y mono
 RUN yum install -y unzip
-RUN yum install -y nuget
 
-RUN nuget update -self
+# --nogpgcheck because nuget-2.12 package is not signed.
+RUN yum install -y nuget --nogpgcheck

tools/dockerfile/distribtest/csharp_jessie_x64/Dockerfile

@@ -40,6 +40,7 @@ RUN apt-get update && apt-get install -y \
     ca-certificates-mono \
     nuget
 
-RUN nuget update -self
+# make sure we have nuget 2.12+ (in case there's an older cached docker image)
+RUN apt-get update && apt-get install -y nuget
 
 RUN apt-get update && apt-get install -y unzip

tools/dockerfile/distribtest/csharp_jessie_x86/Dockerfile

@@ -40,6 +40,7 @@ RUN apt-get update && apt-get install -y \
     ca-certificates-mono \
     nuget
 
-RUN nuget update -self
+# make sure we have nuget 2.12+ (in case there's an older cached docker image)
+RUN apt-get update && apt-get install -y nuget
 
 RUN apt-get update && apt-get install -y unzip

tools/dockerfile/distribtest/csharp_ubuntu1404_x64/Dockerfile

@@ -37,6 +37,7 @@ RUN apt-get update && apt-get install -y \
     ca-certificates-mono \
     nuget
 
-RUN nuget update -self
+# make sure we have nuget 2.12+ (in case there's an older cached docker image)
+RUN apt-get update && apt-get install -y nuget
 
 RUN apt-get update && apt-get install -y unzip

tools/dockerfile/distribtest/csharp_ubuntu1504_x64/Dockerfile

@@ -37,6 +37,7 @@ RUN apt-get update && apt-get install -y \
     ca-certificates-mono \
     nuget
 
-RUN nuget update -self
+# make sure we have nuget 2.12+ (in case there's an older cached docker image)
+RUN apt-get update && apt-get install -y nuget
 
 RUN apt-get update && apt-get install -y unzip

tools/dockerfile/distribtest/csharp_ubuntu1510_x64/Dockerfile

@@ -37,6 +37,7 @@ RUN apt-get update && apt-get install -y \
     ca-certificates-mono \
     nuget
 
-RUN nuget update -self
+# make sure we have nuget 2.12+ (in case there's an older cached docker image)
+RUN apt-get update && apt-get install -y nuget
 
 RUN apt-get update && apt-get install -y unzip

tools/dockerfile/distribtest/csharp_ubuntu1604_x64/Dockerfile

@@ -34,6 +34,9 @@ RUN apt-get update && apt-get install -y \
     ca-certificates-mono \
     nuget
 
-RUN nuget update -self
+# 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
 
 RUN apt-get update && apt-get install -y unzip

tools/dockerfile/distribtest/csharp_wheezy_x64/Dockerfile

@@ -27,6 +27,9 @@
 # (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 # OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-FROM mono:4.2.2.30
+FROM mono:4.4.2.11
+
+# make sure we have nuget 2.12+ (in case there's an older cached docker image)
+RUN apt-get update && apt-get install -y nuget
 
 RUN apt-get update && apt-get install -y unzip