Fix 500 error if 'offset' is out of range

Fix 500 error if 'offset' is out of range

If large value is passed as offset to snapshots, volumes,
backups, consistencygroups and qos_specs list api then it
throws 500 internal server error.

Moved existing validate_integer() method from
cinder.api.openstack.wsgi.Controller to cinder.utils so
that it can also be used for validating offset param for
integer value in _get_offset_param() method and return 400
error if value is out of range.

Please refer:
https://bugs.launchpad.net/cinder/+bug/1535708/comments/1

APIImpact: Return 400 status code if offset is out of range.

Closes-Bug: #1535708
Change-Id: I07a5292645f24c381217d43b71510b3352964b8b