Migrate the rest of the central agent pollsters to use discoveries

Currently, not all the central agent pollsters use discoveries.
In order to enable horizontal scaling, all pollsters must use
discoveries so the resources can be partitioned across the pool
of running central agents.

For pollsters that don't poll specific resources, but only poll
general info from services, we treat the service endpoints as
resources. So in the case of Glance, there will be only one
resource available for all the pollsters (a Glance endpoint).
If there are multiple agents running, only one will be assigned
that endpoint and only one will poll Glance API.

DocImpact
Co-Authored-By: Dina Belova <dbelova@mirantis.com>
Closes-Bug: #1364352
Change-Id: I8f3b228db9aacf3a7cc4b719c50013cc30d5aa79
Merge "Migrate the rest of the central agent pollsters to use discoveries"