Set manila default_share_type config option

Set manila default_share_type config option

Manila default_share_type config option is by default unset.
This option is used by manila when a user creates a new
share and doesn't specify share type explicitly.
Albeit it's not hard requirement to have this option set
to run Manila service, it's convenient to set a default
share type and also it seems to be a general community
opinion that this option should be set.

Note that setting this option does not create the share
type itself (this still has to be done manually which is
probably best because admins may want customize default type
settings according to their needs).

Change-Id: Iab60e42c7f347bbf074d60eb91dd4a1f6a94d3a6
Closes-Bug: #1654204