Description of problem:
Duplicated lines on "Launch Instance" Tab

Version-Release number of selected component (if applicable):
rhos13 z1:
openstack-nova-compute-17.0.3-0.20180420001141.el7ost.noarch

How reproducible:
100%

Steps to Reproduce:
1. Login to the dashboard
2. Click Project->Volumes, select one volume, click "Lanch as Instance",
   click "Source": Select Boot Source: Volume, click "Cancle".
3. Select the volume, click "Lanch as Instance" again. Click "Source",
   you'll see two lines: "Delete Volume on Instance Delete" as boot-from-volume.png

Actual results:
In step3, there are two lines: "Delete Volume on Instance Delete".

Expected results:
In step3, there is only one line: "Delete Volume on Instance Delete".

Additional info:
- boot-from-volume.png