Update help for --nic opt and make net-id or port-id required

Commit updates help string for --nic option to reflect that specifying
net-id and port-id keys are optional but that at least one is
required.  Additionally, we change _boot in novaclient/v1_1/shell.py
to raise an exception if random keys are added to --nic string and if
--nic string does not contain net-id or port-id keys.

Change-Id: Icf94c395bd09160aa6a1b849eb464d72e410e1ae
Fixes: bug #1052356
Fixes: bug #1191139
Merge "Update help for --nic opt and make net-id or port-id required"