Add rally use command - part 2

This completes the behaviour of the 'rally use' command.
when the command is called, it creates a file called openrc-{deployid}
in ~/.rally/ which contains instructions to export deployment specific
environment variables

It also creates a symlink ~/.rally/openrc that refers to the latest
deployment called with 'rally use'

It becomes simple to just do 'source ~/.rally/openrc' and then
'nova list'

Implements blueprint: rally-use-command

Change-Id: I51ddfdbf7b8ab41b5b26685dd7b671468d306fe1