Use correctly /etc/sysconfig and /etc/default env defaults

[arm4b]

There are places for default env variables in different distro families:

• /etc/sysconfig - for Red Hat-like
• /etc/default - for debian-like

We support them in init files:
https://github.com/StackStorm/st2chatops/blob/master/debian/st2chatops.upstart#L24
https://github.com/StackStorm/st2chatops/blob/master/rpm/st2chatops.service#L10

So st2chatops.env should not overwrite env variable, if it was already passed.
The good example is: https://github.com/StackStorm/st2chatops/blob/master/st2chatops.env#L1

export ST2_HOSTNAME="${ST2_HOSTNAME:-localhost}"

^^ Do that for every other env variable in st2chatops.env as a rule.

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Ideally, apart of editing existing st2chatops.env (sed-like stuff is error-prone) user can just place his custom variables (good to do with Configuration Management tools) in /etc/sysconfig/st2chatops or /etc/default/st2chatops and be sure that they're used as a priority.

[cognifloyd]

Currently it is also impossible to override most of the vars in st2chatops.env when running bin/hubot from the command line because of

st2chatops/bin/hubot

Line 15 in 9066a1f

. $HERE/st2chatops.env

In the smoke tests of the ansible-st2 playbook, essentially, we run:

HUBOT_LOG_LEVEL=debug timeout 10 bash -c '(sleep 5; echo exit ) | bin/hubot'

Note that I don't think defining the variables with a bash default is sufficient, even though this it would allow overriding the variables from the command line:

st2chatops/st2chatops.env

Line 1 in 9066a1f

export ST2_HOSTNAME="${ST2_HOSTNAME:-localhost}"

That's not enough because it would make handling those settings through configuration management more error prone and problematic. We need a dedicated configuration file for system-specific settings that overrides the default settings in st2chatops.env. Putting this system-specific config file in the distro-specific environment variables file makes sense to me.

Please change the implementation of bin/hubot to support setting the variables outside of st2chatops.env including from the command line (for testing purposes) and from distro-specific environment variable files.