need more fine-grained control of the creation of the "extraPartition" used by varlibcontainers

[xphyr]

When applying "ImageBasedInstallationConfig" to a server, we need additional control over the creation of the varlibcontainers partition.

Currently functionality only allows for defining an "ExtraPartitionStart" and then consume the remainder of the disk, or you can set "ExtraPartionStart" to a negative number and it works backwards from the end of the disk consuming a fixed amount of space, and leaving the rest for the root partition.

The problem with this is that it does not allow for reserving any extra space on the device for other uses (such as LVM for local storage).

By adding an extra configuration option "ExtraPartitionSize" we can add additional controls around the creation of the varlibcontainers partition, and allow for additional partitions to be defined later via MachineConfigs.

NOTE: adding the additional partition will need to be configured with some form of post config script, or manually via the console until the MCO is updated to allow for changes to the partition configuration (https://github.com/openshift/machine-config-operator/blob/master/docs/FAQ.md#q-can-i-use-the-mco-to-re-partition-or-re-install).

I will be submitting a PR with these changes shortly.