Feature: Ability to use existing API Gateway

Context

Hi, we're a small company in the process of gradually expanding our monolithic elasticbeanstalk application with smaller targeted microservices based on ApiStar / Zappa. We're getting good mileage so far and have deployed a small selection of background task-type services using SNS messaging as triggers, however we'd really like to start adding actual endpoints to our stack that call lambda functions. I've been trying for a couple of days to figure out how to properly setup an API gateway that will proxy to both our existing stack and newer services deployed with zappa. I've tried manually setting up a custom domain for the zappa apps and a cloudfront distro in front of both the existing stack and the new api gateway but am not getting very far. It would be a lot simpler IMO if we could specify an existing API gateway ARN in the zappa_settings.json and elect to use that so that it could integrate with an existing configuration.

Expected Behavior

Would be great if I could specify "api_gateway_arn" in the zappa_settings.json file to use an existing api gateway

Actual Behavior

There is no option to do this

Possible Fix

I guess this is not straightforward given the API gateway is created via a cloudformation template, I guess it'd require moving some of that to boto functions instead ?

Steps to Reproduce

N/A

Your Environment

Zappa version used: 0.46.2
Operating System and Python version: 3.6
The output of pip freeze: ably==1.0.1 apistar==0.5.40 argcomplete==1.9.3 attrs==18.1.0 awscli==1.15.19 backcall==0.1.0 base58==1.0.0 boto3==1.7.0 botocore==1.10.36 cairocffi==0.8.1 CairoSVG==2.1.3 certifi==2018.4.16 cffi==1.11.5 cfn-flip==1.0.3 chardet==3.0.4 click==6.7 colorama==0.3.7 cookies==2.2.1 coverage==4.5.1 cssselect2==0.2.1 decorator==4.3.0 defusedxml==0.5.0 docutils==0.14 durationpy==0.5 et-xmlfile==1.0.1 Faker==0.8.15 Flask==0.12.2 future==0.16.0 hjson==3.0.1 html5lib==1.0.1 humanize==0.5.1 idna==2.7 ipython==6.4.0 ipython-genutils==0.2.0 itsdangerous==0.24 jdcal==1.4 jedi==0.12.0 Jinja2==2.10 jmespath==0.9.3 kappa==0.6.0 lambda-packages==0.20.0 MarkupSafe==1.0 more-itertools==4.1.0 msgpack-python==0.5.6 openpyxl==2.5.3 parso==0.2.1 pdfrw==0.4 pexpect==4.5.0 pickleshare==0.7.4 Pillow==5.1.0 placebo==0.8.1 pluggy==0.6.0 prompt-toolkit==1.0.15 psycopg2==2.7.4 ptyprocess==0.5.2 py==1.5.3 pyasn1==0.4.2 pycparser==2.18 Pygments==2.2.0 PyNaCl==1.2.1 Pyphen==0.9.4 pytest==3.5.1 pytest-cov==2.5.1 pytest-cover==3.0.0 pytest-coverage==0.0 pytest-faker==2.0.0 pytest-mock==1.10.0 -e git+https://github.com/crowdcomms/pytest-sqlalchemy.git@f9c666196d59edd289ecf57704ec7b51cff4440a#egg=pytest_sqlalchemy python-dateutil==2.6.1 python-slugify==1.2.4 PyYAML==3.12 requests==2.19.0 responses==0.9.0 rsa==3.4.2 s3transfer==0.1.13 simplegeneric==0.8.1 six==1.11.0 SQLAlchemy==1.2.7 text-unidecode==1.2 tinycss2==0.6.1 toml==0.9.4 tqdm==4.19.1 traitlets==4.3.2 troposphere==2.3.0 Unidecode==1.0.22 urllib3==1.23 virtualenv==15.2.0 wcwidth==0.1.7 WeasyPrint==0.42.3 webencodings==0.5.1 Werkzeug==0.14.1 whitenoise==3.3.1 wsgi-request-logger==0.4.6 zappa==0.46.2

Miserlou / Zappa