cacert is not picked up correctly by alarm services

related to bug 1266472 which changes client to look only at os_cacert arg.
update alarm services to use just os_cacert as well.

Change-Id: Id28a0838330296fe030fa7c4a35a216dd3e64bad
Closes-Bug: #1266477
tests: allow to skip if no database URL

This is needed for certain driver that we may not want to test if
there's no URL provided in the scenario, i.e. no environment variable
can be found.

Change-Id: I0301987885184ce1e2f83c10db99dba70450449a
Replace mongo aggregation with plain ol' map-reduce

Fixes bug 1262571

Previously, the mongodb storage driver an aggregation pipeline
over the meter collection in order to construct a list of resources
adorned with first & last sample timestamps etc.

However mongodb aggregation framework performs sorting in-memory,
in this case operating over a potentially very large collection.
It is also hardcoded to abort any sorts in an aggregation pipeline
that will consume more than 10% of physical memory, which is
observed in this case.

Now, we avoid the aggregation framework altogether and instead
use an equivalent map-reduce.

Change-Id: Ibef4a95acada411af385ff75ccb36c5724068b59
Merge "Replace mongo aggregation with plain ol' map-reduce"
Merge "cacert is not picked up correctly by alarm services"
Merge "tests: allow to skip if no database URL"