fix all eth* flagged mgmt_itf

[sohorx]

Issue:

I encountered an issue with VLAN interfaces based on eth* interfaces in ifupdown2. Currently, these interfaces are treated differently from other interface types, resulting in MTU discrepancies.

Consider the following configuration:

iface eth1 inet static
    address 1.1.1.1/32
    mtu 2000

iface eth1.1212 inet static
    address 2.2.2.2/32

This results in the eth1.1212 MTU being 1500 instead of 2000.

Management Interface Handling:

What I understand is that interfaces prefixed with "eth" are considered Management Interfaces by ifupdown2 (these interfaces do not have MTU configured resulting in my issue).

Management interface prefixes are configurable in a policy JSON file. However, I believe that ens3 and eth1 interfaces should not have different default handling.

Proposed Changes:

Now, with this PR, "None" is being configured and allowed as the Management Interface prefix, making both ens3 and eth1 behave the same way by default.

Users can then customize their policies to handle eth-prefixed interfaces as they did before.

Additional Suggestion:

Furthermore, I think making a policy using regex might be more useful and less error-prone than a simple "startswith".

[sohorx]

I have updated the PR description in an effort to make it more understandable.

:)