Add subnets as a dependency for router

Because router's external gateway depends on at least one external subnet,
so add subnets as an implicit dependency for the router.

Change-Id: Ib34636b440b3eee3993cd1030dab23312824510d
Closes-Bug:1279645
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
Revert "Add validation to KeyPair resource"

This reverts commit 6124fdea6552145ec38817bbd409b81d717be1c2.

Nova Keypair validation doesn't work with stack-update.  Validation will
fail even if the Nova Keypair resource wasn't changed.

Change-Id: Iaa17e97ca7847c344aabd46f62ae3068cfbbe061
Closes-Bug: #1296747
Don't catch all exceptions in image constraint

Just catch ImageNotFound, as otherwise we can hide other problems.

Change-Id: Ib0e813043b1e4fe7b5b4a14ae8130ac17c4355a5
Closes-Bug: #1297349
Merge "Add subnets as a dependency for router"
Merge "Revert "Add validation to KeyPair resource""
Merge "Don't catch all exceptions in image constraint"
Merge "Change router's prop name agent_id to l3_agent_id"