forall: use smart sync override manifest if it exists

If a workspace is synced with the -s or -t option, the included projects
may be different to those in the original manifest. However, when using
the forall command, the list of the projects from the original manifest
is used.

If the smart sync manifest file exists, use it to override the original
manifest.

Change-Id: Iaefcbe148d2158ac046f158d98bbd8b5a5378ce7