Fix default image name extraction for vSphere.

[fabulous-gopher]

From @prashima on April 13, 2017 23:5

This is failing 'kops create -f FILENAME' (for instance groups) and 'kops upgrade' commands, with following errors:

Create:
I0412 11:08:23.025842 80677 populate_instancegroup_spec.go:257] Cannot set default Image for CloudProvider="vsphere"

Upgrade:
W0413 11:48:52.216116 15456 upgrade_cluster.go:202] No matching images specified in channel; cannot prompt for upgrade

Copied from original issue: vmware-archive#50

[fabulous-gopher]

From @pdhamdhere on April 28, 2017 17:52

@casualjim This could be good one to get started and familiar with KOPS. Please re-assign as appropriate.

[chrislovecnm]

/area vsphere

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