WebHostBuilder should have an option to set the IApplicationEnvironment.ApplicationName

[pranavkm]

We could default this to the StartupAssemblyName \ WebHostOptions.Application. In addition, it would be useful to be able to set these properties independently of each other.

[pranavkm]

More background: In Mvc, we treat the IApplicationEnvironment.ApplicationName as the entry assembly. This is required to make controller discovery work when the actual entry point is the test runner but the test wants Mvc to treat the test assembly as the entry point. It'd be nice to take advantage of the IApplicationEnvironment that the builder creates rather than creating and injecting a mock one in the test,

[davidfowl]

/cc @JunTaoLuo

[muratg]

Looks like a good change. Assigning to @JunTaoLuo

[JunTaoLuo]

Merged to dev