Add general multiplier for find timeouts

Although the timeouts specified in the acceptance tests are enough in
most cases they may not be when running them in a slow system or
environment. For those situations a general multiplier for find
timeouts is added. It can be set in the "behat.yml" configuration file
to increase the timeout used in every find call (except those that used
a timeout of 0, as in those cases the element had to be already present
when finding it and whether the system is slow or not does not change
that).

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>