Update Grafana alpine image and investigate using npm instead of yarn

[matrixik]

Now we are using alpine:3.5 https://github.com/monasca/monasca-docker/blob/81c2676be30e7f9cef348a94f78def291738418a/grafana/Dockerfile#L1 It have old version of nodejs npm that's is not compatible with newer yarn.

Also in https://github.com/monasca/monasca-docker/pull/207#issuecomment-328730349 @timothyb89 suggest to investigate dropping yarn in favour of simple npm usage. There is also pnpm: https://github.com/pnpm/node-package-manager-benchmark

Related: #207