Fix ami lookup 'Name' key error

[josephjohansson]

When looking for an AMI that does not exist on us-east-1 and us-west-1 (have not tested the regions) that belong to amazon the ami lookup function finds some old ARIs or AKIs. See the example below

{'Architecture': 'x86_64', 'CreationDate': '2011-06-24T20:34:25.000Z', 'ImageId': 'ari-e6bc478f', 'ImageLocation': 'ec2-paid-ibm-was-us-east-1/initrd-2.6.16.60-0.30-xen.x86_64.manifest.xml', 'ImageType': 'ramdisk', 'Public': True, 'OwnerId': '470254534024', 'State': 'available', 'BlockDeviceMappings': [], 'Hypervisor': 'xen', 'ImageOwnerAlias': 'amazon', 'RootDeviceType': 'instance-store', 'VirtualizationType': 'paravirtual'}

As you can see the above dict does not have the 'Name' key. This causes line 96 to fail with a Key error such as the below

(Couldn't resolve lookup in variableAppAMI, lookup: ${Lookup<Literal<'ami'> Literal<'owners:amazon name_regex:ubuntu/images/hvm-ssd/ubuntu-xenial-16.04-amd64-server-[.0-9]+'>>}: (<class 'KeyError'>) 'Name')

This is not the expected error message which should be failed (Couldn't resolve lookup in variableAppAMI, lookup: ${Lookup<Literal<'ami'> Literal<'owners:amazon name_regex:ubuntu/images/hvm-ssd/ubuntu-xenial-16.04-amd64-server-[.0-9]+'>>}: (<class 'stacker.lookups.handlers.ami.ImageNotFound'>) Unable to find ec2 image with search string: owners:amazon name_regex:ubuntu/images/hvm-ssd/ubuntu-xenial-16.04-amd64-server-[.0-9]+)

This changes the key access to a .get() with a default return of empty string in order to satisfy the string requirement of re.match().

[russellballestrini]

@josephjohansson thank you for this! Looks good to me!

CC @phobologic

[josephjohansson]

@phobologic

Are we good to merge this?