LBaaS integration with service type framework

The patch makes the following changes:

* adds new attribute of the pool: provider, which is provider name
as it is written in configuration
* adds support for multiple plugin drivers for loadbalancer
* cleans up healthmonitor-related plugin driver API
Drivers should work with healthmonitor associations only
* adds ability to update provider attribute for the pool used
to reassociate pools with new providers in case their providers
were removed from configuration

implements blueprint lbaas-integration-with-service-types

DocImpact

Change-Id: I4295c9bcceb38e60f813d5596af48bd8194c1c9b
Merge "LBaaS integration with service type framework"