Oslo sync to recover from db2 server disconnects

A checkout listener can be added to sqlalchemy to be able to
handle database disconnect pessimistically and avoid an error in
the event of a DB restart or simply a temp connection issue due
to network glitches. At the moment only mysql is enabled with
this feature. This patch enables db2 as well.

Oslo version in change I5563360b43c4b648f5b035099b0e1d2e71d32503

Partial-Bug: #1231657
Change-Id: Idf8b3b11a06790813fed218bdbaf130554f80fa3
Implement the /v2/sample API

This is a simple implementation of the new sample endpoint in the API.
It provides raw accesses to the samples collected by Ceilometer, without
imposing any kind of filtering on the meter name.

The fields are also correctly named this time.

Change-Id: I9e01a082c04e0462811befd98899e2f2d4a4dc82
Blueprint: sample-api
api: raise ClientSideError rather than ValueError

This also enables the error message to be translatable.

Change-Id: If63ba0f7bc04eb490578c7598875407e796700aa
Merge "Oslo sync to recover from db2 server disconnects"