Documentation update

[amercader]

Fixes #198
Fixes #200
Fixes #129

• Updated and simplified documentation, consolidated in the README
• Document all config options and defaults, add support for defining them via env vars
• Document concurrent setup with Postgres + uWSGI threads

[amercader]

@mbocevski @jqnatividad would love your input on this, particularly on the High Availability Setup. @jqnatividad I think I captured all your proposed changes but let me know if you think something is missing.

[mbocevski]

I think it's better the default URL to be http://127.0.0.1:8800 as is in the config files.

Suggested change

	ckan.datapusher.url = http://0.0.0.0:8800/
	ckan.datapusher.url = http://127.0.0.1:8800/

[mbocevski]

When changing the HOST and PORT of datapusher in settings, one must also change them in the uwsgi config as well https://github.com/ckan/datapusher/blob/master/deployment/datapusher-uwsgi.ini#L3.

Suggested change

	If you need to change the host or port, copy `deployment/datapusher_settings.py` to
	`deployment/datapusher_local_settings.py` and modify the file to suit your needs.
	If you need to change the host or port, copy `deployment/datapusher_settings.py` to
	`deployment/datapusher_local_settings.py` and modify the file to suit your needs.
	Make sure to update http setting in `deployment/datapusher-uwsgi.ini` to match your host:port
	setting.

[mbocevski]

@amercader Looks clean and I like moving the docs to the repo itself, I had 2 minor comments. Perhaps it's a good idea to include a section in the README about the uwsgi settings, cause we also are setting uid and gid to www-data there and depending on the system, this user/group combination might not exist. It's enough if we point to users to review the uwsgi config to make sure that it suits their system, prior to starting it.

[amercader]

Thanks @mbocevski I added all your suggestions

[jqnatividad]

Great! This section captures the essence of what I did in the two PRs.

[jqnatividad]

We should change it from /tmp to /var/lib/ckan as @mbocevski suggested.

[abedkhooli]

Was this part of the 2.9.1 patch release? Can we move the /tmp/job_store.db to /var/lib/ckan in 2.9.1 or not yet (package install)?

[amercader]

@abedkhooli datapusher has a separate release cycle than CKAN, so it's not related to the patch releases. Unless I'm missing something you can change the db location to /var/lib/ckan or something else right now as long as the folder is writable by the user running datapusher

[abedkhooli]

@amercader thanks. I moved it to /var/lib/ckan/jobstore/job_store.db but getting datapusher.jobs.HTTPError: <unprintable HTTPError object> after it fetches the csv file, determines field types and saving chunk 0 (last). The jobs table logs an error: internal server error which also shows in UI but no other clue what's going on.