Remove the 'migrate' option from cinder-manage

The 'migrate' option was only appropriate for use migrating
databases for volumes from Nova-Folsom-->Cinder-Folsom.  The
code hasn't been tested since.

Given the above facts, this commit removes the migrate code from
cinder-manage.  The man page for cinder-manage was already
updated to remove the option in a previous commit.

(fixes bug #1187134)

Change-Id: I7bf378cd4b0dd4f6d1bdf475b826a59a31112638
Merge "Remove the 'migrate' option from cinder-manage"