Add config option to enable reporting discard

Add config option to enable reporting discard

The way for a client of Cinder (read: Nova) to know whether or not it
can and should use the discard feature for the block device attachment
is to report back discard=True in the connection info. Previously a
driver would need to be updated to provide this functionality, which is
kind of a pain with so many drivers in Cinder.

This change adds a shared boolean config option called
‘report_discard_supported’ which can be set to enable reporting this
functionality to the caller of initialize_connection.

DocImpact
Change-Id: I078e74583621316bcfe138096507e7d22af0d712
Implements: blueprint discard-config-option