Enabled Libvirt driver to read 'os_command_line' from image properties.

The os_command_line can now be presented to the Libvirt driver through the
glance image property 'os_command_line'.  When this property is present,
the driver uses its value instead of the default. Libvirt uses this
value as either kernel command line or arguments to init (in case of LXC).
DocImpact

bp: enable-libvirt-driver-to-read-kernel-command-line-from-glance-image
Change-Id: Iad53c687053bdb5469baad4331ea55f428c170b0
Merge "Enabled Libvirt driver to read 'os_command_line' from image properties."