Cannot renew when using Testing-In-Production and running from a slot

[jvtroyen]

Extension used: 1.0.6.

We are using Testing-In-production as follows:

• we have 2 slots: "staging" and "live"
• we are not using the Production slot
• we usually serve from the "live" slot (except during deploys)
• we need to run the extension on Production because that's where domain names are defined and certificates should be installed
• when running the extension, it does not find the generated acme challenge file because the file is generated on Production slot, but online verification happens against the "live" slot, which does not contain the file

I took the liberty of making a PullRequest:

• added an extra input-field to the extension, where you enter the name of the TIP-slot (testing-in-production)
• when storing the acme challenge file on Kudu, it takes this new property into account, making sure the file ends up somewhere that it can be found

Thanks for making this extension. I hope you deem this addition worthwhile.