Ensure contib prom docker tags match the installed version

**What**
- For each prometheus and alert manager contrib makefile, ensure that
the docker tag matches the version installed in the dockerfile.  This
will ensure that no one pushes mistmatched versions

Signed-off-by: Lucas Roesler <roesler.lucas@gmail.com>

contrib/alertmanager-armhf/0.16.1/Makefile

@@ -3,8 +3,8 @@ all: ci-armhf-build ci-armhf-push
 
 .PHONY: ci-armhf-build
 ci-armhf-build:
-	docker build -t functions/alertmanager:0.15.0-armhf .
+	docker build -t functions/alertmanager:0.16.1-armhf .
 
 .PHONY: ci-armhf-push
 ci-armhf-push:
-	docker push functions/alertmanager:0.15.0-armhf
+	docker push functions/alertmanager:0.16.1-armhf

contrib/prometheus-armhf/2.2/Makefile

 build:
-	docker build -t alexellis2/prometheus:2.3.1-armhf .
+	docker build -t alexellis2/prometheus:2.2.0-armhf .
 

contrib/prometheus-armhf/2.7.1/Makefile

@@ -3,8 +3,8 @@ all: ci-armhf-build ci-armhf-push
 
 .PHONY: ci-armhf-build
 ci-armhf-build:
-	docker build -t functions/prometheus:2.7.0-armhf .
+	docker build -t functions/prometheus:2.7.1-armhf .
 
 .PHONY: ci-armhf-push
 ci-armhf-push:
-	docker push functions/prometheus:2.7.0-armhf
+	docker push functions/prometheus:2.7.1-armhf