Allow configuring email for Openscanhub notifications

[lbarcziova]

Currently, we (packit) receive the email about the progress of the OpenScanHub scan submitted by us. This can be adjusted by utilising --email-to option of osh-cli when submitting the scan.

TODO:

• [ ] add a new configuration option (optional) which users can use to receive emails themselves
• [ ] utilise this when submitting the scan
• [ ] document