Fix #1728 by introducing per-service extras_require

[slafs]

This is the initial approach of fixing #1728 i.e. resolve the dependencies overhead and allow for installing minimal set of dependencies for a given service (e.g. when using only s3 backend, install dependencies used by moto.s3 module only).

Things to discuss/decide:

• is this approach even feasible for @spulec
• do we still need botocore.awsrequest and ConnectionCls fix in moto/__init__.py (why do we need it anyway)?
• do we want to include ALL services in extras_require (in setup.py) i.e. even those that don't need any additional requirements (future-proof)?
• can we trim down more dependencies (i.e. remove any from install_requires still, e.g. werkzeug or Jinja2)?

[coveralls]

Coverage decreased (-0.003%) to 94.372% when pulling 2c8d126 on slafs:moto-1728/dependencies-overhead-backwards-compat into 2433d64 on spulec:master.

[slafs]

@spulec I'd love some feedback/guidance on this approach.

[spulec]

Thanks for opening this! I think this approach makes sense.

For questions 2 and 3, I think the answer is no.

RE trim down more dependencies: probably. If you want to do that, happy to accept it, but also think we could merge with something like this.

[davidszotten]

@slafs are you still interested? are you planning to update this patch (at least to resolve the conflicts)? (if not i might take a stab)

[spulec]

Closing in favor of #3281

Thanks for the help!