qa/tasks/cephadm: override container image tags

qa/tasks/cephadm: override container image tags

By default, container image name is tagged by branch name or
by sha1, i.e. corresponding values suffixed ':' are appended
to the image value.

This patch allows to override the tagging just by providing
desired tag with the standard notation using ':' separator
in the container image parameter value of cephadm task.

Fixes: https://tracker.ceph.com/issues/71844
Signed-off-by: Kyr Shatskyy <kyrylo.shatskyy@clyso.com>