Add default volume type flag

Add default volume type flag

Where using volume type scheduler, 'volume type' is an essential input for
scheduler to perform sophysticated scheduling.  This patch adds a new
flag 'default_volume_type to allow admin to set one volume type as the
default type for volume creation if user doesn't specify any.

Also add more clear 404 message to API if create request provided non-existing
volume type.

Note: Setting the 'default_volume_type' flag to non-existing volume type
is problematic and should be avoided, and the outcome of such behavior is
equivlant to setting default volume type to None.

This is part of patch set implementing 'volume-type-scheduler'.

Change-Id: Ib2b2bbdc9bc96ab097cca2821c9aef04c65c2857