enable multiple collector workers

currently, a single worker thread writes received samples into
database. this is fine for most occasions but during message
spikes, it is not enough. this patch increase amount of workers to
half the number of processors (so we can ensure ceilometer maintains
lightweight design)

this will also help validate mulitple worker setup works.

Partial-Bug: #1336755
Change-Id: I33037f2f9ebfda43238e5cd0cff75adbed65d055
Merge "enable multiple collector workers"