Separate the command for replace_port to delete and add

Separate the command for replace_port to delete and add

When a port has been added to router namespace, trying to replace the
port by adding del-port and add-port in one command, will not bring
the new port to kernel. Even if the port is updated in ovs db and can
be found on br-int, system can't see the new port. This will break
the following actions, which will manipulate the new port by ip
commands. A mail list has been filed to discuss this issue at [1].

The problem here will break the scenario that namespace is deleted
unexpectedly, and l3-agent tries to rebuild the namespace at restart.

Separating replace_port to two commands: del-port and add-port,
matches the original logic and has been verified that it can resolve
the problem here.

[1] http://openvswitch.org/pipermail/discuss/2015-December/019667.html

Change-Id: If36bcf5a0cccb667f3087aea1e9ea9f20eb3a563
Closes-Bug: #1519926