We cannot use Elemental-cli InstallSpec struct to generate config.yaml.
Because there are not all entry with "omitempty", it will cause
generated config.yaml will have several empty entry but didn't be
omitted.
Elemental-cli will treat those empty entry as user-specific value and
override the default value.

Use ElementalInstallSpec to ensure that all entry will be "omitempty".

Signed-off-by: Date Huang <date.huang@suse.com>

Signed-off-by: Date Huang <tjjh89017@hotmail.com>