feat(keycloak): support for relative path and management relative path (#982)

When these parameters were set, the functions get_url() and
get_management_url() did return the wrong URLs (without the prefix)

Now the implementation checks if the parameter is checked and adds it to
the returned URL

This solves
https://github.com/testcontainers/testcontainers-python/issues/964

Co-authored-by: Baitinger Jens <jens.baitinger@de.bosch.com>
Co-authored-by: David Ankin <daveankin@gmail.com>