Added resources support in pollster's interface

1. Change the pollster's interface to support resources:
- Added optional parameter 'resources' to the method get_samples. It
  specifies a list of the endpoints from which the pollster would get
  data.

2. Change the PollingTask to record the resources for each pollster.

3. Pass the resources recorded in PollingTask to the pollsters in
central agent.

This patch only changes the interface of pollsters and PollingTask in
central agent. The existing central agent's pollsters still ignore the
resources passed in by the PollingTask.

Implements: blueprint support-resources-pipeline-item
Change-Id: Ic7a83c63ea291d0527004113bc8d8bb09311d14f
Merge "Added resources support in pollster's interface"