Adding Y value unit into linechart popup

- Adding a simplified solution of fetching a meter unit.
- Adding unit into linechart popup, (Rickshaw doesn't support label
  of axis, we may add it in the future, depending on new wireframes).

Fixes bug 1226489

Change-Id: I01aa27578b30e6ad0d27956a3ddd76663f7549ce
Update error messeages in Launch Instance

If no image is selected when launching an instance, we will get
an error message "There are no image sources available; you must
first create an image before attempting to launch an instance."
In most cases this message is displayed when a user forget to
select an image. This commit changes the error message more generic
to avoid unnecessary confusion.

Closes-Bug: #1225755

It also changes the menu to display "No images available" if
no images are available rather than "Selct image" with no choice.

Change-Id: Ie5dd0760349c8b030baa2b2465776d0fa6c1767f
Merge "Update error messeages in Launch Instance"
Merge "Adding Y value unit into linechart popup"