Change router's prop name agent_id to l3_agent_id

Currently, the OS::Neutron::Router supports associating the L3Agent via
"agent_id" property.

However, Neutron supports a few kind of agents(L3, DHCP and etc).
The property name of "agent_id" is little bit ambiguous.

It's nice to change property name "agent_id" to "l3_agent_id".

Change-Id: Ie3c90d703df81bbf9d541a7d7c08641eed9c8e2a
Closes-bug: #1296608
Make OS::Nova::Server networks property updatable

This patch adds the ability to update the networks property.
Update process consists of two simple steps:
    1. detaching all old interfaces
    2. attaching all new interfaces.

Change-Id: Iaabf8be1adc6cb7d5ecc08edb92d9f9ce029b125
Closes-bug: #1250645
Merge "Change router's prop name agent_id to l3_agent_id"
Merge "Make OS::Nova::Server networks property updatable"