Consistent handling of choiceField for backups

CHanged the way we initially populate the restore
backups choice field in the trove launch dialog
so that it is consistent with other choice fields
in horizon dashboard e.g. compute->instances

An empty choiceField now says
'No backups available'
and a populated choiceFIeld will have an initial
selection of
'Select backup'

Change-Id: I1b0f57656bd649ad92268789f35c9ba77223dbc9
Closes-bug: #1340867
Merge "Consistent handling of choiceField for backups"