wip
additional test diagnostics
Refactor of tests in fences/network/configure_test.go
split security test from restarting variant
Name bridges appropriately
Add standard prefix to bridge interface names

The standard prefix is the Garden network interface prefix, which
consists of "w" followed by the tag.

This ensures that outbound packets from bridges are passed to the
filter forward chain by the following rule in bin/net.sh:

iptables -w -A FORWARD -i ${GARDEN_NETWORK_INTERFACE_PREFIX}+ \
    --jump ${filter_forward_chain}