In order to get a more precise usage of the instances we switched to
using the 'cpu_util' measures provided by gnocchi instead of simply
looking at the 'started_at' and 'ended_at' dates. Besides providing more
granularity to the usage, using this metric also allows taking into
consideration periods of time when the instance wasn't powered on.

Added "Resource flavor" column.

See merge request !3

This is a new column in the costs page that indicates the flavor name or
volume type of a given resource.

Changed how the costs interval is handled.

See merge request !2

This should solve multiple issues that currently appear when the data is
displayed (such as negative costs).

Add missing packages to charm hook.

See merge request !1

Fixes 'ImportError: No module named MySQLdb' error in apache2/error.log
by installing libmysqlclient-dev and mysql-python.

A new panel was added in order to display
project metrics values and their corresponding
price on a specific time interval.