Updated description of create volume type dialog

The existing description is not very helpful, it does not metion
the extra specs that users need to deal with in order to make the
volume type usable.

Updated the description to guide users to set up the extra specs
in Horizon UI.

Closes-Bug: #1325726

Change-Id: I68db21cb45e6fa787ae4845aa8690ab6185dbed8
Merge "Updated description of create volume type dialog"