plugins: Use utf-8 encoding in html report

Use utf-8 encoding to write the HTML report to avoid errors on systems
with different localization.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
Merge pull request #273 from ldoktor/htmlreport

plugins: Use utf-8 encoding in html report
no changes, just style cleanups
avocado: Insert children using add_child tree

When initializing TreeNode we should use the add_child function
in order to set the parent correctly.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
RFC: avocado: Construct the tree directly from yaml file

Instead of creating OrderedDicts and then converting it into
tree structure, this modifies the `mapping` constructor to create
the tree structure directly. This way we only walk through the yaml
file once.

Also remove the `tree_unittest.TestReadYAML` selftest as it lost it's
meaning. Valures are checked in the TestTreeNode selftest.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>