Change I8b43624b934fadf1036fafa1f5e2b94b5317f81c introduced reading
volume_type['extra specs'] from filter_properties in CapacityFilter
without taking into account that volume_type can be None (when
default_volume_type is not set in cinder.conf). This commit fixes that.
Closes-Bug: 1619246
Change-Id: Icfc6ba5f47c3a61b49cbec5110ef7df05eb47195