feat(instance_image) : add resource for instance image

[Mia-Cross]

In response to this issue, we will be adding a new resource to the Terraform provider : instance images.

[Mia-Cross]

Supported attributes for instance images are :

• Zone
• Name
• Root volume
• Architecture
• Default bootscript (keep in update ?)
• Extra_volumes (create + read ok, only update left to implement)
• Project
• Tags
• Public
• Creation date
• Modification date
• From server
• Organization
• State

Images can be made from a snapshot of :

• a server
• a block storage volume
• a server with a block storage volume (depends on extra_volumes)
• a server with a local storage volume (depends on extra_volumes)
• a combination of block storage volumes
• a server with combination of both block and local storage (depends on extra_volumes)

[Mia-Cross]

@remyleone can you checkout the test "TestAccScalewayInstanceImage_SeveralVolumes" ? The weird behavior happens at the 5th and 6th checks concerning additional_volumes. It says "pass" although the part where I set the additional_volumes in the CREATE function is commented and I don't even mention them in the READ function so there's no way they are set.
I tried the debugger but it doesn't seem to step into any function so I don't understand what's happening...