Add --only-vca flag to charmed installer

It will help feature 10239, in order to automate the deployment of the
VCA without needing to deploy OSM.

When the option is enabled in the installation, in the end a string will
be printed out with the command to be executed in OSM to register the
installed VCA.

Change-Id: I1b284a5a650b50e89c07a6d08507406823c6cc2d
Signed-off-by: David Garcia <david.garcia@canonical.com>