Exit w/ valid code when no servers are deleted.

This change updates do_delete in v1_1/shell.py to keep track of
deletion failures and raises an exception when all of the specified
servers cannot be deleted.  This in turn causes nova client to exit
with a correct exit code when no successful deletes occur.

Change-Id: I16ee7a4c754cf2e8add09a41becbcc37edc767ff
Fixes: bug #1185009
Merge "Exit w/ valid code when no servers are deleted."