Add ability to install .NET Core CLI

[PaulStovell]

I really like this plugin, looks like a great way forward for building DNX apps in TeamCity.

The first hurdle I ran into is that you have to install DNU/DNX as the user that the agent is running under. It would remove so much friction if TeamCity could take care of this - either by letting users specify a custom version (the same way they can specify which NuGet executables to use) or just downloading and installing it for them.

To get it to work for ASP.NET 5 RC1, I had to:

• Make TeamCity build agent run as a custom user
• Log in to Windows as that custom user
• Install the command line DNVM
• Run dnvm install latest
• Run dnvm upgrade
• Restart TeamCity build agent to pick up the environment variable changes

[dtretyakov]

@PaulStovell, we will try to automate tools installtion in the next versions of plugin.

Please note, that Microsoft announced that they move from DNX to .NET CLI tools and DNX toolkit will be retired soon.

[dtretyakov]

Now dotnet .core can be installed via scripts as follows.

[dtretyakov]

Since .NET CLI preview3 we can install additional CLI tools:
https://docs.microsoft.com/en-us/dotnet/articles/core/preview3/tools/dotnet-install-script