Fix the RRD RRA’s to collect the correct amount of data for the Previous Period view for each resolution.

Applied when RRD's are created.

RRA:AVERAGE:0.5:1:1200         = 20 hours of 1 minute data
RRA:AVERAGE:0.5:5:720           = 2.5 days of 5 minute data
RRA:AVERAGE:0.5:60:1860       = 2.5 months of 1 hour data
RRA:AVERAGE:0.5:1440:3652   = 10 years of 1 day data
Merge pull request #676 from N0YB/RRD_RRA_Sized_for_Previous_Period

Fix the RRD RRA’s to collect the correct amount of data for the Previous Period view for each resolution.