webadmin: [Fix] Template without migration downtime

Template is created with no default value for migration downtime,
the screen does not close, it requires data.

Change-Id: I478ad01793a9ff22f18d22eb775527252a2ca4b4
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1066617
Signed-off-by: Gustavo Pedrosa <gustavo.pedrosa@eldorado.org.br>