Added support for resource price changes.

See merge request !7

When calculating the costs for a given resource, all the rows in the
price table for that resource are going to be taken into consideration.
The most recent price is going to be displayed in the costs page.

Fix dropdowns and charts UI issues

See merge request !5

Most notably, fixes issues where dropdowns can't be used and charts
aren't displayed.

The issues were caused by some javascript files being added multiple
times by Horizon's JS static list (HORIZON_CONFIG.js_files) builder. As
a solution, the JS script builder algorithm was overwritten.

Changed the way the instance usage is calculated.

See merge request !4

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.