FIX: Initialize L2BD keyword

Currently in Performance the "Initialize L2 bridge domain" keyword is adding
interfaces to L2DB per node and at the same time waiting until interfaces
become UP. This may be an issue if physical interfaces interconnecting DUTs
required to be administratively UP on both DUTs to physical link state became
UP.

This fix is supposed to check the link state after all DUTs are
initialized.

Change-Id: I430b4285967f6168e6c74e0089a3e0d037b251d1
Signed-off-by: Peter Mikus <pmikus@cisco.com>