Use master as default for debian upgrade.

Make sure that uri is defined for debian upgrades.
Use master as default.

Added _get_uri_() which consolidates checking and assignment of
sha1, tags, and branch values.

Fixes: 7707
Signed-off-by: Warren Usui <warren.usui@inktank.com>
Merge pull request #254 from ceph/wip-7707-wusui

Use master as default for debian upgrade.