Cleanup leftover wizard data in session on adding App to Env

Default horizon setting is to use cookie-based session storage, which
has a limit of 4096 bytes (actually, a bit less), thus it can be
overflown very quickly in case one user adds many applications.

Closes-Bug: #1372963
Change-Id: Icf5a908fa330c6a80f80c31bc59331735e3a86d4