Fix inconsistencies in devices import/export - #4401

Hello,

Second wave of fixes. Sorry if I should have made a second PR, but GH puts my new commits in this PR too...

• Issue#3: When exporting and re-importing devices, all fields were considered as modified even if the value was the same.
• Issue#4: When importing devices, the hostname is checked against a regex. This prevent re-importing previously exported data that were created via the GUI which is not checking the hostname.
• Issue#5: When importing a device with no type and no section, these fields are not mandatory, but they are checked against existing types and sections which fails and prevent importing.
• Issue#6: A bug was introduced in my previous commit which was not removing the trailing ';' on device sections. This was causing imported devices to belong to a non existing "empty" section.

I'm proposing the following fixes:

• Issue#3: I've fixed this.
• Issue#4: To harmonize the process, I propose not to check device hostname on import like it's done when creating a device on GUI. Maybe it would have been better to introduce device hostname check in GUI too ? What do you think.
• Issue#5: I've added a second check to not verify if "empty" section and type exists and allow device import in that case
• Issue#6: I've fixed this.

Seems to fix the following Issues: #3887 #2103 #1894 #3463

Thanks,

A.