Refactor to remove _recycle_ip

Since _recycle_ip is now just a pass-through to _delete_ip_allocation
it can be removed.

Change-Id: Ifba3da902de599f748038a33ef3bd98ff77c22b9
Closes-Bug: #1269501
tests/unit: Initialize core plugin in TestL3GwModeMixin

TestL3GwModeMixin can fail randomly because it doesn't initialize
core_plugin and can be run random core plugin depending on execution
order of tests. It also fails with core plugin uninitialized when it
is run without other tests.

This patch refactors the setup code of core plugin and apply it to the
related tests.
This patch reveled the same bug of test_metaplugin.py which is also
fixed by this patch.

Closes-bug: #1273259
Change-Id: I3c1d4d8b3d69262b89c7747daa8267bf2c8e7f6b
Merge "Refactor to remove _recycle_ip"
Merge "tests/unit: Initialize core plugin in TestL3GwModeMixin"