sort snapshots in drop-down when launching instance

Closes-Bug: #1329949
Change-Id: I6e20406b2e6f0dd6248233333bccf932d0350b92
Add extra_context to SetInstanceDetailsAction.get_help_text

Add extra_context in the SetInstanceDetailsAction.get_help_text in the
panel project.instances. This argument was missing in the child class.

Change-Id: I3a766eaa7c1aeb7ed0479d53d32fc01f8a41dc75
Closes-Bug: #1334294
Merge "Add extra_context to SetInstanceDetailsAction.get_help_text"
Merge "sort snapshots in drop-down when launching instance"