Create snapshots when min-ready is >= 0

Currently, if min-ready is 0 a snapshot will not be created (nodepool
considers the image to be disabled). Now if min-ready is greater than or
equal 0, nodepool will create the snapshot.  The reason for the change is to
allow jenkins slaves to be offline waiting for a new job to be submitted. Once
a new job is submit, nodepool will properly launch a slave node from the
snapshot.

Additionally, min-ready is now optional and defaults to 2. If min-ready is -1
the snapshot will not be created (label becomes disabled).

Closes-Bug: #1299172

Change-Id: I7094a76b09266c00c0290d84ae0a39b6c2d16215
Signed-off-by: Paul Belanger <paul.belanger@polybeacon.com>
Merge "Create snapshots when min-ready is >= 0"