docker / midPoint_container

Although the tests are quite advanced, they are not really complete yet.
Most notably, group membership update in LDAP is not checked now.
We should include running Trigger Scanner in these tests eventually.

$tag is now defined in common.bash and used throughout building scripts.
The goal is to allow building and testing both from Jenkins and standalone.

build.sh is now more generic; allows rebuilding if -r is specified.

...and added cleanup step as well

The container is now built directly from the root directory.
Even the simplest composition was moved to demo/plain.