Fix the change_suffix Makefile target allowing the user to just change
the URL suffix of the ingresses for Prometheus, Alertmanager and
Grafana.
This way it's not required to rebuild all manifests from scratch. Also
improved the Readme with this instructions.
I have tried to run a simple `make deploy`. First problem was my GOPATH was blank (even though the golang was installed). After that was fixed, I needed to actually run `make vendor` before I could even consider running `make deploy`. I also needed to add my GOPATH/bin directory to path as it did not find gojsontoyaml otherwise
`export PATH=$PATH:$GOPATH/bin`
Error I was solving:
```
$ make deploy
Installing jsonnet
rm -rf manifests
./scripts/build.sh main.jsonnet /home/xzy/cluster-monitoring/bin/jsonnet
using jsonnet from arg
+ set -o pipefail
+ rm -rf manifests
+ mkdir manifests
+ /home/xyz/cluster-monitoring/bin/jsonnet -J vendor -m manifests main.jsonnet
+ xargs '-I{}' sh -c 'cat {} | gojsontoyaml > {}.yaml; rm -f {}' -- '{}'
RUNTIME ERROR: couldn't open import "kube-prometheus/kube-prometheus.libsonnet": no match locally or in the Jsonnet library paths
main.jsonnet:12:13-63 thunk <kp> from <$>
main.jsonnet:30:81-83
<std>:1268:24-25 thunk from <function <anonymous>>
<std>:1268:5-33 function <anonymous>
main.jsonnet:30:64-99 $
During evaluation
make: *** [Makefile:12: manifests] Error 1
```
