Use nova network-create --bridge for FlatDHCP

Although it's not documented anywhere, the --bridge-interface option is
only used by the VLAN network manager, but this page explicitly
uses FlatDHCP, so the --bridge option is appropriate.

Backport of commit 781929333f55eee4ddb131440ced315e271f976b from master

Change-Id: I2bb13ba53cdc09171f16862f6915687a07de2150
Merge "Use nova network-create --bridge for FlatDHCP" into stable/havana