Set is_temporary flag for formats of ephemeral spaces

Before the patch, when ephemeral space was created flag
is_temporary was set after space was actually created.
Which in turn lead to corresponding flag of tuple_format
being set to `false`.
So, having heavy load using ephemeral spaces (almost any
SQL query) and snapshotting at the same time might lead
to OOM, since tuples of ephemeral spaces were not marked
as temporary and were not gc-ed.
Patch sets the flag in space definition.