Make methods better descriptive

Calling 'Sellable.cancel' to make it available is one of the most weird things
one can possibly imagine. Also, 'Sellable.can_sell' looks like it would return
a True if it can be sold and False if not. Actually it would set the sellable
to available status without any check.

To avoid those problems, make those methods better descriptive. Change 'cancel'
and 'can_sell' to a single 'set_available' and the 'can_be_sold' to
'is_available' (that one for consistency) so they are just like all other
status' api implemented elsewhere.

Change-Id: I9f3a9a6601fc5627f975a6596e6078d6ecebfda9
Merge "Make methods better descriptive"