Skipping central agent pollster when keystone not available

When keystone is not available, instead of skipping all central agent
pollsters, we should let the pollsters decide whether to skip or not,
because not all central agent pollsters requires keystone.

Closes-Bug: #1316532

Change-Id: Ib34f9838bd128e1b357937f0c177d8a09f19b13f
Merge "Skipping central agent pollster when keystone not available"