Work around kernel issues via kernel boot params

Ubuntu 12.04 kernels appear to have some known I/O issues that can
cause kernel stack traces and I/O errors in certain conditions.
One operation that appears to trigger the problem is cloning a cinder
volume when using the default iSCSI backend.  Telling the kernel
to use the deadline elevator (instead of the default CFQ elevator)
appears to effectively prevent the kernel problems from being
triggered.  The deadline elevator can be enabled by passing the
kernel a parameter at boot time.

This commit provides a method for users to specify parameters to
be passed to the kernel at boot time via site.pp and by default
passes the default Ubuntu options plus the elevator change needed to
prevent issues with cloning Cinder volumes.

Closes-Bug: #1212250