Add configurable API url to Watson TTS

[krisgesling]

Description

The new IBM Watson TTS service now provides a unique url for your TTS
API resource. This adds a configuration option to override the default
url. As the url provided by IBM contains the api_path, we check for it
and if found remove it, as the RemoteTTS.__request method appends it
when making the call.

How to test

Create an IBM cloud account and add Watson TTS as described in the docs.
Add a url attribute to your config using the url provided with your API key. This can be added with or without the API path "/v1/synthesize", and with or without trailing slashes.

Contributor license agreement signed?

• CLA (Whether you have signed a CLA - Contributor Licensing Agreement

[pep8speaks]

Hello @krisgesling! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

There are currently no PEP 8 issues detected in this Pull Request. Cheers! 🍻

Comment last updated at 2020-03-26 06:13:04 UTC

[forslund]

The remove_trailing_slash is suggested for removal in #2496, so maybe replace that with a simpler `.rstrip('/')

I also wonder if we want to do some generalization, allowing all remoteTTS to specify a custom url using the same config parameter, by changing the self.url = url line in remote_tts.py to self.url = config.get("url", self.url).

I'm also wondering if the remoteTTS could use some extra customizability... but I'll add a separate issue for that.

[krisgesling]

Yeah good suggestion re generalization. This seems like a good candidate.

Wondering if it's worth checking for the url.endswith(api_path) in remote_tts.py too?
The only case it would break for is if an API was in the form api.com/endpoint/endpoint
hence the url == 'api.com/endpoint',
and api_path == '/endpoint'
and the url would then get stripped to become just api.com

This seems like a very remote case.

[krisgesling]

Maybe I'll leave it where it is for now, and as you said we can consider further changes to remote_tts in a later PR.

[forslund]

Looks great!