ansible: Fix merge_vars.yml

ansible: Fix merge_vars.yml

Currently, we override the variables in merge_vars. However, if we run
the script several times (e.g. a host is a grafana and a collectd node)
then vars[item] is defined but it can be that it is not a mapping. In
this case we redefine the value to empty string breaking the actual
values of the variables.

We redefine e.g. devel_mode or use_epel to empty string this way making
it false for the grafana server node that is both grafana and the
collectd node.

Signed-off-by: Boris Ranto <branto@redhat.com>