Add Ubuntu 19.04 images for 3.0

This only applies to 3.0 and will not be back ported to 2.x
Once 19.10 is released, we will add 19.10 images. 19.04 images will be supported for 3 more months before being dropped. The same thing will happen once 20.04 (LTS) is released - 20.04 images will be added and the 19.10 images will be supported for 3 more months before being dropped.
Support will remain for Bionic (18.04) throughout the lifetime of .NET Core 3.0.

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