Improve 'dotnet new sln' message

[balachir]

CLI VERSION

REPRO STEPS
dotnet new sln

ACTUAL

EXPECTED
The message seemed a little odd - I was expecting something along the lines of "A Solution File has been created". I can see that this is just a generic message format used for all the templates. I wonder if this can / should be improved.

[mlorbetske]

Maybe we could add a custom "Hello" message to the template

[ErikSchierboom]

I would argue that the current message looks like a solution template was created, not that a solution was created using a template.

[sayedihashimi]

It would be good to allow templates to specify the "Hello" message.