travis: Enable arm64 builds

Setup a travis test matrix on a new variable 'TARGET', which specifies the CI
target.  Update the script section with a conditional that runs the needed
commands for each target.

Also, set go_import_path to make cloned repos work, enable the trusty VM, and
enable verbose builds when testing.

Signed-off-by: Geoff Levand <geoff@infradead.org>

test

@@ -17,7 +17,7 @@ GOSIMPLE_UNUSED_PATHS=$(go list ./... | sed -e 's/github.com\/coreos\/etcd\///g'
 # Invoke ./cover for HTML output
 COVER=${COVER:-"-cover"}
 
-GO_BUILD_FLAGS=-a
+GO_BUILD_FLAGS="-a -v"
 source ./build
 
 # Set up gopath so tests use vendored dependencies