Build upon changes in kolla which change strategy of installing projects
in containers when in dev mode. This fixes problems where when package
file manifest changes, the changes were not reflected in to
devmode-enabled container.

It changes the strategy of installing projects in dev mode in containers.
Instead of bind mounting the project's git repository to the venv
of the container, the repository is bind mounted to
/dev-mode/<project_name> from which the it is installed using pip
on every startup of the container using kolla_install_projects script.

Also updates docs to reflect the changes.

Depends-On: https://review.opendev.org/c/openstack/kolla/+/925712


Closes-Bug: #1814515
Singed-off-by: Roman Krček <roman.krcek@tietoevry.com>
Change-Id: If191cd0e3fcf362ee058549a1b6c244d109b6d9a