VSTS IsHostedAgent detection is flawed

When running CakeBuild in VSTS and rely on `Context.TFBuild().IsRunningOnVSTS`. However we have 2 hosted build agents and second build agent is called "Hosted Agent 2". And build is always failing on the second agent.

[Here](https://github.com/cake-build/cake/blob/d8f7fe179bdf41fa9d2dbc426a3bd6fbcef7dcfc/src/Cake.Common/Build/TFBuild/TFBuildProvider.cs#L82) code insists that agent name is exactly "Hosted Agent".

Also We are setting up private build agent and [this code](https://github.com/cake-build/cake/blob/d8f7fe179bdf41fa9d2dbc426a3bd6fbcef7dcfc/src/Cake.Common/Build/TFBuild/TFBuildProvider.cs#L45) does not agree with private build agents. It is still VSTS build, only not run in hosted agent.

I propose to change the check for the agent name to `.StartsWith("Hosted Agent")` and remove check for `IsHostedAgent` from `IsRunningOnVSTS` option.

@agc93 what do you think? (as far as I remember you have done work on this)

I can do PR for this.