Linux Agent improvements for L3

Linux Agent improvements for L3

prereq for bp quantum-l3-fw-nat

- make init_l3 take cidrs, rather than assuming an augmented port object
- make namespace for agent operations optional and allow the namespace
name to be configured.
- allow plug() operation to take an optional bridge parameter indicating
which bridge to plug into
- add namespace support for iptables manager
- make OVS plug() set the IP address, etc. of a device even if it already
exists.

Change-Id: Id4fec9bf7cda30c45b94eccd25e9e54dc5af97b7