Update README to the valid Oslo-incubator doc

Currently, the README in `openstack/common/` contains a link
to a blank page.
This patch changes the link to a valid documentation page.

Change-Id: I6c7a7a5fcef76f49b5d38c79b10a2a849046a32f
Closes-Bug: #1268051
Set image size to None after removing all locations

Based on current design, the image size should be updated to
None if there is no location assocaited with the image. This
patch fixes this issue and introduces some related changes.

Closes-Bug: #1263729

Change-Id: I893468f1dc320ea9434f07c3a32f978cd5941b33
Prevent min_disk and min_ram from being negative

At the moment, in both api versions, min_disk and min_ram can be set to
negative value - which doesn't make sense. This commit fixes this behavior
in the two versions of the glance api.

Change-Id: Ib7855c1d928ab8262dd2a109619a4e3d2299ff9e
Closes-bug: #1265711
Merge "Set image size to None after removing all locations"
Merge "Prevent min_disk and min_ram from being negative"
Merge "Update README to the valid Oslo-incubator doc"