avocado.core.tree: Match whole key in filters.

When filtering nodes, use the whole key for comparing.

So if you have /long and /longest and you're filtering for /long,
then it will match /long alone.

The previous behaviour was using the partial
string match, so both /long and /longest were being returned,
which is wrong.

Signed-off-by: Rudá Moura <rmoura@redhat.com>
Merge pull request #237 from ruda/multiplex_filters_match_whole_key

avocado.core.tree: Match whole key in filters.