Fix import keypair help text

The existing text was just copied from the create keypair view.

This commit adds a bunch more help text and instructions for creating
an SSH keypair from a terminal.

Change-Id: I43cb31ac0fcf46dbd1c607687810caf0e18ffde4
Closes-bug: #1237264.
(cherry picked from commit abdab25f03098394c57304f28b011bc6d893e9c5)