Add configuration via env vars

[fkromer]

Allows to override scrapyd.conf config values http_port, bind_address, username, password via env vars SCRAPYD_HTTP_PORT, SCRAPYD_BIND_ADDRESS, SCRAPYD_USERNAME, SCRAPYD_PASSWORD.

[codecov[bot]]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (00e30f8) 87.20% compared to head (36383ef) 87.00%.

Files	Patch %	Lines
scrapyd/app.py	55.55%	4 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #499 +/- ## ========================================== - Coverage 87.20% 87.00% -0.20% ========================================== Files 41 41 Lines 1883 1893 +10 ========================================== + Hits 1642 1647 +5 - Misses 241 246 +5 ``` | [Flag](https://app.codecov.io/gh/scrapy/scrapyd/pull/499/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scrapy) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/scrapy/scrapyd/pull/499/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scrapy) | `87.00% <55.55%> (-0.20%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scrapy#carryforward-flags-in-the-pull-request-comment) to find out more.

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[jpmckinney]

Done in c6fd8834a1c44ccb38aa123e139be5ae56298d5f