fix: make the interfaces up after configuration

[gatici]

Description

Fixes the Network is unreachable problem which is described below by making the interfaces up after UPF network configuration.

Env is an EC2 instance. Two additional interfaces attached for access and core.
Configuration passed to the charm included interface name, interface IP and gateway IP.
Network interfaces are configured correctly, but both are `DOWN`. 
Charm code should make sure that the interfaces have been brought UP after changing configuration.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that validate the behaviour of the software
• I validated that new and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have bumped the version of the library