Fix webex teams config

[jacobtomlinson]

Description

Looks like there was a bug in the voluptuous config for the WebEx Teams connector. The URL option is a function and needs to be called.

https://github.com/alecthomas/voluptuous#urls

Fixes #1501

Status

READY

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

• Added unit test

Checklist:

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[codecov]

Codecov Report

Merging #1504 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1504   +/-   ##
=======================================
  Coverage   99.90%   99.90%           
=======================================
  Files          52       52           
  Lines        3296     3296           
=======================================
  Hits         3293     3293           
  Misses          3        3           

Impacted Files	Coverage Δ
opsdroid/connector/webexteams/__init__.py	100.00% <100.00%> (ø)

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