separate the plugin code into backend specific part and generic part

[YAMAMOTO Takashi]

currently the plugin and db mixin have some backend specific logic hardcoded.
it's better to separate them into their own classes, or a service driver.
so that generic part can be used by other implementations.

examples of such logic:
- agent rpc (TaasAgentApi, TaasCallbacks)
- vlan id management (TapIdAssociation)

an example of an alternative backend, which doesn't need those logic:
https://review.openstack.org/#/c/301843/
it's sdn-controller backed (vs agent) and uses a different
encapsulation (non-vlan)

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/307708

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/307708
Committed: https://git.openstack.org/cgit/openstack/tap-as-a-service/commit/?id=5035aa4a57e7641e6bb202f61ded18c5956e705c
Submitter: Jenkins
Branch: master

commit 5035aa4a57e7641e6bb202f61ded18c5956e705c
Author: Yoichiro Iura <email address hidden>
Date: Tue Apr 19 09:33:45 2016 +0000

    Support service framework to separate plugin logic

    This fix supports service framework in order to separate
    the plugin code into backend specific and generic part.

    Change-Id: I66d0f4a9ce5453ec7ca0bf0fe52f24a3a2e3c8ea
    Closes-Bug: #1567770