Add new cloud-init-datasources element

This moves cloud-init data source configuration to a general purpose
cloud-init-datasources element that can be used to explicitly configure
the list of cloud-init sources that will be queried on first boot.

cloud-init-nocloud now depends on this new element to configure the
datasource_list while continuing to prep the image for a nocloud first boot.

Change-Id: Ibcc3b86d6ca567a23f89b7a1a36bc713e444ef68
Closes-bug: #1316475
Check return code of element-info

Ensure the return code of element-info is actually checked by moving the
export IMAGE_ELEMENT to the subsequent line.

A recent change (Ic0a061995e2ae708c95a535cba6237bff58f7d93) exported the
IMAGE_ELEMENT variable. This caused the return code of element-info to not
actually be checked, since the export command is now checked instead and
almost always will return 0.

This caused a misleading error message when using diskimage-builder with an
unknown element since the script didn't exit when it should have:
$ bin/disk-image-create -a amd64 foobar vm
Building elements: base  foobar vm
ERROR: Element 'foobar' not found in 'bin/../elements'
Expanded element dependencies to:
Building in /var/tmp/image.DuIDY1lW
Please include at least one distribution root element.

Change-Id: I3d9c02bf0b1a4f320280185dd3fab0697d2c92f2
Only use Ec2 cloud-init data source for Ubuntu

Default to only having cloud-init query Ec2 on first boot for Ubuntu,
until cloud-init has been SRU'd to fix the CloudSigma data source issue
that causes Trusty boots to hang.

Change-Id: Icb3734d5ae78f4a0a6c0fae1af4a2ce3c809308c
Partial-bug: #1316475
Reinstate Trusty as default for Ubuntu

Now that issues associated with this bump have been addressed
elsewhere, reinstate trusty as the default DIB_RELEASE for
the ubuntu element.  Also adds a note about memory requirements
to the ramdisk element.

Change-Id: Ia89c83ba0ee4ad3bd89b833ba0db08135eac63ed
Merge "Add new cloud-init-datasources element"
Merge "Only use Ec2 cloud-init data source for Ubuntu"
Merge "Check return code of element-info"
Merge "Reinstate Trusty as default for Ubuntu"