Protect against /etc/nodepool not existing
Protect against /etc/nodepool not existing

To transition from images without /etc/nodepool to images with,
check if /etc/nodepool exists before writing anything there.
Later, this change can be removed once we can assume it always
exists.

Change-Id: Ia39653cbbcea68cad618351bb61a39190a67e8b6

citrix-openstack/nodepool,seandst/nodepool,Tesora/tesora-nodepool,citrix-openstack/nodepool,seandst/nodepool,Tesora/tesora-nodepool