change the log format in Python and Node.js services.
Effected services are currencyservice, emailservice, paymentservice,
and recommendationservice. Loadgenerator is left as is because of
the diffculty to change the log format and log target in locust.
ref. #47
Reduce docker image for emailservice to ~240 MB (down from ~ 1.31 GB)
Main application (`email_server.py`) now runs as python 2.7. Before we had both Python 2.7 and Python 3 installed in the image.
Switched to using `python:2.7-alpine3.8` as the base image, and used multi-stage dockerfiles to keep dependencies minimal.
Fixes#49
From my shell:
```
$ docker build -t emailservice:dev . && docker run -it emailservice:dev
Sending build context to Docker daemon 97.28kB
Step 1/17 : FROM python:2.7-alpine3.8 as base
---> b2bc7255b42c
Step 2/17 : FROM base as builder
---> b2bc7255b42c
Step 3/17 : RUN apk add --update --no-cache gcc linux-headers make musl-dev python-dev g++ cairo-dev cairo openssl-dev gobject-introspection-dev
---> Using cache
---> 6daf3d9fe49a
Step 4/17 : ENV GRPC_PYTHON_VERSION 1.15.0
---> Using cache
---> 3e33d97d9580
Step 5/17 : RUN python -m pip install --upgrade pip
---> Using cache
---> e8fa3879c282
Step 6/17 : RUN pip install grpcio==${GRPC_PYTHON_VERSION} grpcio-tools==${GRPC_PYTHON_VERSION}
---> Using cache
---> c6fba7743eed
Step 7/17 : COPY requirements.txt .
---> Using cache
---> 1f6b0a444980
Step 8/17 : RUN pip install -r requirements.txt
---> Using cache
---> 8cc0a7af6aa8
Step 9/17 : FROM base as final
---> b2bc7255b42c
Step 10/17 : RUN GRPC_HEALTH_PROBE_VERSION=v0.1.0-alpha.1 && wget -qO/bin/grpc_health_probe https://github.com/grpc-ecosystem/grpc-health-probe/releases/download/${GRPC_HEALTH_PROBE_VERSION}/grpc_health_probe-linux-amd64 && chmod +x /bin/grpc_health_probe
---> Using cache
---> e954a0384081
Step 11/17 : ENV PYTHONUNBUFFERED=0
---> Using cache
---> 64ece3d72a66
Step 12/17 : WORKDIR /email_server
---> Using cache
---> 27b34dc14492
Step 13/17 : COPY --from=builder /usr/local/lib/python2.7/ /usr/local/lib/python2.7/
---> Using cache
---> 60035ec8dfd4
Step 14/17 : RUN apk add --no-cache libstdc++
---> Using cache
---> 920be90c126e
Step 15/17 : COPY . .
---> Using cache
---> 9541bed2d7a0
Step 16/17 : EXPOSE 8080
---> Using cache
---> 48fbeaa852b9
Step 17/17 : ENTRYPOINT [ "python", "email_server.py" ]
---> Using cache
---> ff317770992d
Successfully built ff317770992d
Successfully tagged emailservice:dev
starting the email service in dummy mode.
listening on port: 8080
```
