Mostly integration test work:

    * Switches A1's three test receivers (integration tests) over to golang; this was mostly done to learn the go xapp framework and they are identical in functionality.
    * Upgrades the version of rmr in A1 and all integration receivers to 1.13.*
    * Uses a much fancier Docker build to reduce the size of a1's image. The python:3.7-alpine image itself is 98MB and A1 is now only ~116MB, so we're done optimizing A1's container size.

Issue-ID: RICAPP-61
Change-Id: Ic2004a5b457c55c730575aa8326e61b01e8546da
Signed-off-by: Tommy Carpenter <tc677g@att.com>