Use ugettext_lazy instead of ugettext.

We should use ugettext_lazy to let users in different languages
get correct translation.Though in views or similar places
we can use ugettext with no problems, we should make code
style consistent.

This also fix the issues that the django proxy objects appear on the
launching instance page instead of translated strings. Now the
translated quota strings are handled correctly.

Fixes: bug #1178722

Change-Id: I76f0d31e6546d70e5ece7e86a06f7eefd04861d2