Inherit base image properties on instance creation

When creating an instance, a few properties from the image will be saved
in the instance's system_metadata under the prefix 'image_' for latter
usage.

The keys that the instance will inherit from the image are: 'min_ram',
'min_disk', 'disk_format' and 'container_format' together with the
inheritable image properties.

Related to bug #1039662

Change-Id: I358fb6d8dcafd67c289fe5083bb82140f3759fee
Add methods to get image metadata from instance

This patch adds a couple of utility functions that enclose all the logic
for getting and parsing the image metadata stored in the instance's
system metadata.

First, this will try to fetch the metadata from the real image and will
prevent it from failing if it is not available. It will be then merged
with the image metadata stored during the instance creation.

Related to bug #1039662

Change-Id: I2130caf19858585571b1199e27f0a98ad5f08701
Avoid errors on some actions when image not usable

Using the metadata saved on instance creation, we can now get all the
image related metadata we need from the instance itself.

This patch replace the logic for getting the image metadata on some
actions that shouldn't fail when the image is not accessible (create
an snapshot, resize, migrate, rescue an instance or attach an
interface).

Fixes bug #1039662

Change-Id: Id8c2f13ca96908317d271a84fd685104b64f87e9