Updated for offline install

Issue: ascom-au/install-iso#3

Signed-off-by: Nathan Callister <ncallister@iwsoftware.com.au>
Added required sources

Issue: ascom-au/install-iso#3

Signed-off-by: Nathan Callister <ncallister@iwsoftware.com.au>
Prevented cache update in offline mode.

Issue: ascom-au/install-iso#3

Signed-off-by: Nathan Callister <ncallister@iwsoftware.com.au>
Added GPG keys for package sources

Issue: ascom-au/install-iso#3

Signed-off-by: Nathan Callister <ncallister@iwsoftware.com.au>
Updated yaml block syntax to strip chomping

Issue: ascom-au/install-iso#3

Signed-off-by: Nathan Callister <ncallister@iwsoftware.com.au>
Merge branch 'install-iso#3'

Issue: ascom-au/install-iso#3

Signed-off-by: Nathan Callister <ncallister@iwsoftware.com.au>