Add ability to set Prometheus url with custom path without trailing "/"

[Laci21]

Fixes #193.

What

Join URL paths so that custom path is taken into account.

Additional context

With this change, I was able to install Flagger like this:

helm upgrade -i flagger flagger/flagger \
--namespace=istio-system \
--set metricsServer=http://prometheus.istio-system.svc.cluster.local/prometheus

... and execute a canary deployment based on the docs successfully.

It seems to also work with custom paths with trailing slashes (prometheus/) or without setting any custom paths.

[stefanprodan]

LGTM

Thanks @Laci21