handles keyboard interrupt

When an user intentionally provides an keyboard interrupt, heatclient
throws the entire traceback on to the terminal instead of handling it.

heatclient will now handle the keyboard interrrupt and provides an crisp
message on to the terminal.

Change-Id: Iaa30048e7948d7db1d5b256dd59ca2e0dd842df9
Closes-Bug: #1367283
Merge "handles keyboard interrupt"