Nethvoice Proxy: add backup and restore procedure

[Amygos]

The backup procedure must include:

• Backup of the configured routes
• Backup of the configured trunks
• Backup of the Nethvoice Proxy configuration

The restore procedure must include:

• Restore of the configured routes
• Restore of the configured trunks
• Restore of the Nethvoice Proxy configuration if applicable

Right now the replacement of an existing Nethvoice Proxy module instance with a
previously backed up version is not possible. The user must first remove the
current instance and then restore the backup on the selected node.