Fix the default text for Instance Boot Source

This change removes the leading and trailing '---' from the default
text of the Instance Boot Source dropdown in the Launch Instance dialog.
This change makes the default text more consistent with other dialogs.

The localized files (django.po) were not modified because they will be generated
from transifex eventually.

Change-Id: I92bf8b8aa52724cbb851933132ee33bc87cd68b5
Closes-Bug: #1322934
Merge "Fix the default text for Instance Boot Source"