Ensure that the user/project exist on alarm update

This patch ensure that the user and project exists before updating the
alarm.

Closes bug #1260776

Change-Id: I25cf55d91b78a32bda671426972b7f418bf85e21
Ensure the correct error message is displayed

When a untranslated error message pass through the translation pecan
hook, the previous translated error is send to the client.

This patch fix that by reseting the "translatable_error" of the
translation pecan hook before each request.

Change-Id: Icee79cf09fd696655048c7c0cf8a3d6706e31701
Fixes: bug #1260398
Add metadata query validation limitation

There is currently no way to pre-determine the schema of
resource_metadata. So it will be hard to validate when
running a query. This patch includes this limitation in
ceilometer documentation.

Change-Id: I804467c3bb8e7d336ae881b25434063839951724
Partial-Bug: #1245367
Merge "Add metadata query validation limitation"
Merge "Ensure that the user/project exist on alarm update"
Merge "Ensure the correct error message is displayed"