Rename resize-flavor subcommand to resize-instance

The trove resize-flavor <instance> <flavor_id> subcommand actually
changes the flavor of an instance, not the size of the flavor itself.

To improve clarity, the resize-flavor subcommand was renamed as
resize-instance.

To avoid backward compatibility issues, marked deprecated the
resize-flavor subcommand while we introduce the new resize-instance
subcommand.

DocImpact: This potentially affects any user scripts that may
exist.  Any instructions like:
'trove resize-flavor <instance> <flavor_id>'
should be changed to:
'trove resize-instance <instance> <flavor_id>'

Change-Id: Ie801d5632383b67c3f777487f4b6bc608b4a69e8
Closes-Bug: #1324719