removed unused context in check_*** methods

Input parameter 'context' of Check_*** methods(check_attach,
check_detach, _check_metadata_properties, _check_volume_availability)
in cinder/volume/api.py are not being used, so remove this 'context'
parameter.

Change-Id: I314f662f2fe54a6b073181ff5c0f0214e9ef822a
Closes-bug: #1265725
Merge "removed unused context in check_*** methods"