Rename some options, make image-os-type option mandatory for knife-openstack and rspec tests for same change
removed unwanted code from openstack_helper
Implemented review comments to abstract out validate params and use set_image_os_type
Merge pull request #52 from opscode/adamed-oc-9368

OC-9368:  image_os_type option should be compulsory in knife-openstack.
Implemented changes to set valid exit codes for knife-openstack
Updated rspec tests
Implemented review comments to send error message
Merge pull request #54 from opscode/adamed-oc-9390

OC-9390: Handle exit of knife plugin, with correct exit status and from common place in code