Add TOCI_UNDERCLOUD_EXTRA_ELEMENTS

Add a new option which can be used to configure extra elements used
for the undercloud image. Useful for ad-hoc testing of elements for
undercloud images.

Change-Id: Ic5a7cf4c1f3ff1f5e1879097622837b1cc92267c
Add TOCI_OVERCLOUD_EXTRA_ELEMENTS

Add a new option which can be used to configure extra elements used
for overcloud images. Useful for ad-hoc testing of elements
for overcloud images.

Change-Id: I873cb756e8f8c62ccc87a36a1f0ef9d9f5fcca4d
Add TOCI_SEED_EXTRA_ELEMENTS

Add a new option which can be used to configure extra elements used
for the seed image. Useful for ad-hoc testing of elements for seed
images.

Change-Id: I4fe29bce25b743f15d573feed255f0ebc03532c7