ofagent: Remove @author tags and update copyright notices

When writing these modules, ofagent team (kakuma, yamamoto) thought
that @author tag was OpenStack's style to represent authorships.
But it turned out to be wrong.  Acutally there seems to be a consensus
not to use them.  So, replace them back to the original Ryu-style
copyright notices.

Change-Id: Icc9fdd1850c0c1dbd354bb3312199a7f6849925a
Fix AttributeError when setting external gateway on DVR router

DVR routers will have this manager initialized only after one
or more subnets have been attached to the router. To address
the issue, make sure the manager is defined and handle the snat
rules appropriately.

This patch also makes _update_arp_entry more defensive; this is
because the arp update process can be affected by the same issue:
the router may not have internal ports at the time the request
come in. This is likely when VM's port creation and router
configuration overlap slightly.

Closes-bug: #1353006

Change-Id: Ib46852f5b264e5ef2e2d499d3351f8974e393011
Co-authored-by: Rajeev Grover <rajeev.grover@hp.com>
Merge "Fix AttributeError when setting external gateway on DVR router"
Merge "ofagent: Remove @author tags and update copyright notices"