Fix(eos_designs): remove the need for mgmt_gateway

Change Summary

This PR relax the need for a mgmt_gateway. Currently the behavior is that eos_designs will still try to install a static default route to the mgmt_gateway even if it is not defined.

Note: however the default behavior is still to configure a default MGMT vrf so this PR may need some more adjustments.

Related Issue(s)

Fixes #1886

Component(s) name

arista.avd.eos_designs

Proposed changes

in the eos_designs/template/base/static-routes.j2, attempt to install a default mgmt route only if mgmt_gateway is defined.

How to test

molecule with a new device no_mgmt_interface

Checklist

Repository Checklist

• [x] My code has been rebased from devel before I start
• [x] I have read the CONTRIBUTING document.
• [x] My change requires a change to the documentation and documentation have been updated accordingly.
• [x] I have updated molecule CI testing accordingly. (check the box if not applicable)