Explicit image selection

[Ian Booth]

Hi

As evidenced by inquiries to this list and elsewhere, there's a user desire to
be able to explicitly specify the image id they want to run. Normally, for EC2
and Openstack at least, the correct image based on series and tools architecture
is selected by looking at cloud image metadata files. Additionally, in the case
of Openstack, the user can set up their config with a fallback default-image-id
attribute since image metadata may not always be available. In the EC2 case, it
is expected a suitable image will always be able to be found hence there's no
need nor allowance for a default id.

Allowing the users to explicitly select the image id they want to run can of
course lead to problems. But some users require that functionality.

Proposal:

Introduce a new config attribute called "user-image-id".
If set, this short circuits the image id lookup from metadata.

The existing default-image-id attribute as implemented for Openstack only would
remain unchanged, and is still used as a fallback when all else fails (if
specified). The difference is that this new attribute overrides the image id
lookup and would be documented with suitable warnings etc.

Sound reasonable?