Count management interface in CLAB_INTFS_WITH_MGMT env var

[hellt]

We now introduce a new env var CLAB_INTFS_WITH_MGMT that counts the management interface in when counting calculating the number of interfaces required by clab.

The reason for this change is to make sure that

1. we always check that the management interface is available, not only clab interfaces
2. we respect the network mode: none setting when eth0 interface is added manually to the list of endpoints and therefore it shouldn't be implied as an addition to datapath interfaces

Goes together with vrnetlab change srl-labs/vrnetlab#166

** This is a breaking change for vrnetlab nodes**
Because newer vrnetlab (>=0.15.0) nodes will rely on the new CLAB_INTFS_WITH_MGMT env var, we will add an early error when this env var is not found asking users to upgrade containerlab.

Older vrnetlab nodes will continue to function because they would still use the CLAB_INTFS env var and since the calculation logic for CLAB_INTFS hasn't changed it will work as before.
It will not though support the network mode: none.

fix #1851

[hellt]

not a prio right now