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
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 "Prevent min_disk and min_ram from being negative"
Merge "Update README to the valid Oslo-incubator doc"