Mqtt use tls

[fmeef]

The MQTT_NO_TLS variable is a negative condition, where every other cmake variable is a positive condition. Change MQTT_NO_TLS to MQTT_USE_TLS to avoid confusion.

[codecov-io]

Codecov Report

Merging #428 into master will not change coverage.
The diff coverage is n/a.

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[fmeef]

I accidentally merged in changes from master, which broke some tests in travis. It is fixed now and should work.

[redboltz]

@gnu3ra , thank you for sending the PR. I have two comments.

Conclict

It seems that the PR conflict with the master. Could you rebase the PR from the up to date master? At that time, please squash commit message. You can do git push --force ....

Consistency vs Zero configuration by default

At first, mqtt_cpp has only two configuration macros. They are MQTT_USE_WS and MQTT_NO_TLS. If they aren't set, WebSocket is not supported and TLS is supported by default.
I think that the default setting is good for typical *1 usecase. MQTT is usually used in IoT domain. I guess that the message is sent via the internet so TLS support is required. If some user that has secure network (e.g.VPN), TLS isn't required. They can define MQTT_NO_TLS.

However now we have many configuration macros and all of them except MQTT_NO_TLS are positive meaning. As you mentioned, MQTT_NO_TLS is confusing.

Now we are doing breaking change for v7.0.0. I think that it is a good time to update TLS support macro. I'd like to hear @jonesmz 's opinion.

*1 I know the definition of 'typical` is difficult.

[jonesmz]

This line of code:

OPTION(MQTT_USE_TLS "Disable building TLS code" ON)

(Btw @gnu3ra that comment may not be accurate any longer)

sets the default setting for this CMake option to "ON", which means that, unless specified on the commandline by another flag, mqtt_cpp would be compiled with TLS support.

Note though that the current setting

OPTION(MQTT_NO_TLS "Disable building TLS code" ON)

Appears to default the library to not include TLS, because it's defaulting to "enable" "NO_TLS"

---

Personally, I think it would be better to default MQTT_USE_TLS = off, so that mqtt_cpp has fewer dependencies for a "default" build, but I don't have a strong opinion on it.

[redboltz]

@jonesmz , thank you for the comment. Let's replace MQTT_NO_TLS with MQTT_USE_TLS.
And the default value of MQTT_USE_TLS is OFF.

@gnu3ra , could you (force) update the PR?

[fmeef]

Merge conflicts are resolved, and MQTT_USE_SSL defaults to no. Does this work?

[redboltz]

Thanks! merged.