Merge pull request #146 from BnLG/snom

Snom enhancements

Hi,
This PR fixes a bug when Snom devices are using SRTP : on-hold calls which are resumed do not have sound anymore / are silent. Disabling user_auth_tag option corrects this.
https://service.snom.com/display/wiki/user_auth_tag
Note that Yealink phones also use a 80-bit auth-tag.
While here, let's be sure to enforce SRTP when used, thanks to user_savp option.
Also, let's set the setting_server option, so that it's not left blank in the web UI of the phone.
Let's then correct the BLF keys behavior, setting the quick_transfer to attended.
https://service.snom.com/display/wiki/quick_transfer
Snom behavior is now the same as on Yealink devices.
Finally, let's prevent Snom devices from rebooting when using the "Synchronize" Portal button.
https://service.snom.com/display/wiki/How+to+trigger+the+phone+to+reset+or+synchronize+its+settings+via+provisioning+-+mass+deployment
Note that even with reboot=false, devices will reboot if a provisioned option needs a reboot to apply.
reboot=false will allow to apply more easily (without impact) most of the options such as accounts, BLF etc...
Snom behavior is now the same as on Yealink devices here too.
Many thanks 👍

Reviewed-by: Alexandre Fournier <None>