Exit expirer earlier if db-ttl is disabled

Now, the ceilometer-expirer exit without doing the backend db request if
the db-ttl is disabled.

Fixes bug #1270760

Change-Id: I49580be14cd932317ff30070aeec391ef456712e
Set the SQL Float precision

By default mysql use a precision of 23 to store a Float and
sqlite/postgresql use 53.

The patch set the precision of Float to 53 to force all sql backends to
use the same double precision.

Change-Id: I36680ed180999e401e45428af068fd32126247da
Exclude weak datapoints from alarm threshold evaluation

Addresses: BP exclude-low-sample-count

To avoid alarm threshold evaluation being inappropriately skewed by
aggregates with anomalously low sample count, these weak data may be
excluded for selected alarms by setting a new 'exclude_outliers'
field on the alarm threshold rule.

An outlier in this context is considered to be an aggregate with
sample count that differs from the observed mean by more than two
standard deviations.

Change-Id: I45d05db57662588df5796a8d6864ed01d1e4165f
Merge "Set the SQL Float precision"
Merge "Exit expirer earlier if db-ttl is disabled"
Merge "Exclude weak datapoints from alarm threshold evaluation"