Implement simple global variables system in docs and add global variables
KOLLA_OPENSTACK_RELEASE, KOLLA_ANSIBLE_MAJOR_VERSION  and
KOLLA_BRANCH_NAME.

Closes-Bug: #1844710
Change-Id: I2c131463eaf1c51eb8768f6d90e9697c77a7833b
Signed-off-by: Adrian Andreias <adrian@fleio.com>