This adapts manifests to be in line of using git reference
for refering to global/secrets repos instead of versioned folders.

Change-Id: I0100fb90666e335bd99d304b3804c180b25a9d16
Depends-On: https://review.openstack.org/#/c/577886/

Introducing additional manifest layer that can be used for
custom overrides that are not captured by global, type, and site.

Immediate need is overrides required by ariship-seaworthy.

Change-Id: I3a09e14213f26ca156c0bd79f3e1766926ae44c2

- global/type manifests
 - airship-seaworthy site manifests

Change-Id: I5951bba063e6447ff0d2e1b40d9711209919f7d1