Publish AZ SIG images

[sayanchowdhury]

To be merged with https://github.com/flatcar/jenkins-os/pull/371

The script publishes vhd images to Azure Shared Image Galleries

Pending Item:

• Need to move the download into the az_sig_publish. lbzip2 is not available in package repos. Also, need to check if curl works.
• replication to target regions is still not working. need to look into
• remove cleanup items before merge.
• update default values before merge

CI http://jenkins.infra.kinvolk.io:8080/job/container/job/az_sig_publish/98/consoleFull

[chewi]

Nice work, thanks.

Just a general tip, although Gentoo is admittedly quote-heavy in its ebuilds, you don't need to quote:

• When assigning FOO=${} or FOO=$()
• Inside [[ ]] unless it's a variable on the right-hand-side.
• case ${FOO} in