Modify some help and error messages in ec2creds identityv2
Modify some help and error messages in ec2creds identityv2

Usually we use "(s)" to show about multi deletion in
help message. In addition, I think "EC2 credentials"
is better than "EC2 keys" in the error message.

Change-Id: I6a6461291542701d87a55d9ea0ea1fda6db04601

dtroyer/python-openstackclient,dtroyer/python-openstackclient,openstack/python-openstackclient,openstack/python-openstackclient