acs-engine code is using older key for azure-cli install

[radhikagupta5]

In latest master the Docker file is installing latest az-cli using,

RUN apt-get update
&& apt-get install apt-transport-https
&& echo "deb [arch=amd64] https://packages.microsoft.com/repos/azure-cli/ wheezy main" > /etc/apt/sources.list.d/azure-cli.list
&& apt-key adv --keyserver packages.microsoft.com --recv-keys 52E16F86FEE04B979B07E28DB02C46DF417A0893
&& apt-get update
&& apt-get install azure-cli

According to this document below the key used above will be deprecated in May 2018 and we should start using the new key,
https://docs.microsoft.com/en-us/cli/azure/install-azure-cli-apt?view=azure-cli-latest