fixing Azure AppService on get PythonVersion

[AlisonVilelaTenchi]

Fix this error

Cannot read property 'substr' of undefined

The code was trying to read linuxFxVersion in the wrong way

[CLAassistant]

All committers have signed the CLA.

[asieira]

This PR fixes the following error we were getting when running the recently updated Azure code against a live environment:

2020-06-03 21:52:57/app/scans/plugins/azure/appservice/pythonVersion.js:69
2020-06-03 21:52:57var pythonVersion = webConfigs.linuxFxVersion.substr(webConfigs.linuxFxVersion.indexOf('|') + 1);
2020-06-03 21:52:57^
2020-06-03 21:52:57TypeError: Cannot read property 'substr' of undefined
2020-06-03 21:52:57at /app/scans/plugins/azure/appservice/pythonVersion.js:69:71
2020-06-03 21:52:57at Array.forEach (<anonymous>)
2020-06-03 21:52:57at /app/scans/plugins/azure/appservice/pythonVersion.js:53:26
2020-06-03 21:52:57at /app/scans/node_modules/async/dist/async.js:3110:16
2020-06-03 21:52:57at eachOfArrayLike (/app/scans/node_modules/async/dist/async.js:1069:9)
2020-06-03 21:52:57at eachOf (/app/scans/node_modules/async/dist/async.js:1117:5)
2020-06-03 21:52:57at Object.eachLimit (/app/scans/node_modules/async/dist/async.js:3172:5)
2020-06-03 21:52:57at Object.run (/app/scans/plugins/azure/appservice/pythonVersion.js:32:15)
2020-06-03 21:52:57at /app/scans/engine.js:164:24
2020-06-03 21:52:57at /app/scans/node_modules/async/dist/async.js:3682:9