add mesos basic auth

[andreaspeters]

This PR will add the basic auth method to authenticate against the mesos master. Then the nice DC/OS net is also useful for "normal" mesos installations without authentication token. The authentication string have to be in a env variable and should looks like MESOS_BASIC_AUTH="Basic <BASE64STRING>".

[mesosphere-ci]

Can one of the admins verify this patch?

[codecov]

Codecov Report

Merging #169 into master will decrease coverage by 0.01%.
The diff coverage is 80%.

@@            Coverage Diff             @@
##           master     #169      +/-   ##
==========================================
- Coverage   67.94%   67.92%   -0.02%     
==========================================
  Files          55       55              
  Lines        3584     3588       +4     
==========================================
+ Hits         2435     2437       +2     
- Misses       1149     1151       +2

Flag	Coverage Δ
#ct	48.24% <80%> (ø)	⬆️
#ct2	48.24% <80%> (ø)	⬆️
#eunit	24.52% <80%> (+0.05%)	⬆️

Impacted Files	Coverage Δ
apps/dcos_net/src/dcos_net_mesos.erl	74.07% <80%> (+0.04%)	⬆️
apps/dcos_l4lb/src/dcos_l4lb_mgr.erl	82.56% <0%> (-0.33%)	⬇️

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[ichernetsky]

@andreaspeters, thanks a lot for your contribution!

It is preferred not to supply individual configuration values through environment variables. Please refer to https://github.com/dcos/dcos-net/blob/master/apps/dcos_net/src/dcos_net_app.erl#L45 to learn more about the way dcos-net gets configured.

By default, dcos-net looks for configuration files under /opt/mesosphere/etc/dcos-net.config.d, which can be overwritten, for instance, through its application environment variable: config_dir. Please note that configuration files should be written in the Erlang term format.

[andreaspeters]

Hi @ichernetsky , thanks for your response.
Actually, I'm not a Erlang dev, I just copy 'n paste what I already found in your code. And you already use a lot of os:getenv(""). :-) Well, for me it would not be easy to change it like u prefer. But now, you know and I think you understand why I wish me to have basic auth in dcos-net. If someone can add this feature (with a better peace of code), I would be very happy about it. :-)