Change type of Cluster.AllocSolution

Originally, this data type was used both by instance allocation (1
result), and by instance relocation (many results, one per
instance). As such, the field 'asSolutions' was a list, and the
various code paths checked whether the length of the list matches the
current mode. This is very ugly, as we can't guarantee this matching
via the type system; hence the FIXME in the code.

However, commit 6804faa0 removed the instance evacuation code, and thus
we now always use just one allocation solution. Hence we can change
the data type to a simply Maybe type, and get rid of many 'otherwise
barf out' conditions.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Agata Murawska <agatamurawska@google.com>