connection can fail if database.connection is not defined

[gordon chung]

we added support for a split db environment (metering_connection, event_connection...) but still support database.connection as our fall back option.

there's a bug in our connection logic where if you don't define a purpose it will assume you are connect to metering db but it only defaults to use database.connection option and does not check metering_connection option.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/154222

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/154222
Committed: https://git.openstack.org/cgit/openstack/ceilometer/commit/?id=f80cd71d86011b968bd141161369fd9ce27ddcd6
Submitter: Jenkins
Branch: master

commit f80cd71d86011b968bd141161369fd9ce27ddcd6
Author: gordon chung <email address hidden>
Date: Mon Feb 9 15:07:04 2015 -0500

    check metering_connection attribute by default

    ensure we check metering_connection and only default to
    database.connection if missing.

    Change-Id: I985f66c145bc8795bc93f846413823c4fea2a6ac
    Closes-Bug: #1419962