Add locals_base option

This option allows you to define 'locals' style dependencies as relative
to a $locals_base variable, similar to how $tarballs_base works. This
can help you avoid defining the same hard-coded path in kolla-build, and
instead pass it to kolla via a flag.

Similar to $tarballs_base, this works via oslo_config's existing
interpolation functionality[1].

Example usage:
  [DEFAULT]
  locals_base = /opt/openstack

  [neutron-base]
  type = local
  location = $locals_base/neutron/neutron-stable-rocky.tar.gz

[1]:
https://docs.openstack.org/oslo.config/latest/reference/configuration-files.html#option-value-interpolation

Change-Id: I8670db4dde4dac26df28502637eadc6b854261cf

doc/source/admin/image-building.rst

@@ -149,6 +149,19 @@ The ``kolla-build.conf`` file could look like this:
    Note that the name of the section should exactly match the image name
    you are trying to change source location for.
 
+If using the ``local`` source type, the ``--locals-base`` flag can be used to
+define a path prefix, which you can reference in the config.
+
+.. path etc/kolla/kolla-build.conf
+.. code-block:: ini
+
+  [DEFAULTS]
+  locals_base = /home/kolla/src
+
+  [heat-base]
+  type = local
+  location = $locals_base/heat
+
 .. _dockerfile-customisation:
 
 Dockerfile customisation

kolla/common/config.py

@@ -168,6 +168,8 @@ _CLI_OPTS = [
                 help='Show image dependencies (filtering supported)'),
     cfg.BoolOpt('list-images',
                 help='Show all available images (filtering supported)'),
+    cfg.StrOpt('locals-base', default='./',
+               help='Base directory for local source resolution'),
     cfg.StrOpt('namespace', short='n', default='kolla',
                help='The Docker namespace name'),
     cfg.StrOpt('network_mode', default='host',