XmlTemplates: do not drop toplevel aliases so that python task can see them

Current implementation uses XmlTemplates to process aliases in the recipe XML
during parsing phase. This works fine for tasks written in XML itself or
xincluded. For the Python tasks this currently does not work since the task
is not being processed for aliases. This patch prevents removal of available
top-level aliases from XmlTemplates processor so that python task can access
them via ControllerAPI during the task execution.

Signed-off-by: Jan Tluka <jtluka@redhat.com>
Signed-off-by: Jiri Pirko <jiri@resnulli.us>
NetTestController: adding get_alias method

Adding _get_alias method to the NetTestController.

Signed-off-by: Jan Tluka <jtluka@redhat.com>
Signed-off-by: Jiri Pirko <jiri@resnulli.us>
ControllerAPI: adding get_alias method to ControllerAPI

ControllerAPI now provides get_alias() method to get alias defined in the
recipe XML.

Example:

my_value = ctl.get_alias("my_alias")

The code above gets the value of the alias named "my_alias" previously defined
in <lnstrecipe> namespace, e.g:

<lnstrecipe>
    <define>
        <alias name="my_alias" value="my_value"/>
    </define>
    ...
</lnstrecipe>

Signed-off-by: Jan Tluka <jtluka@redhat.com>
Signed-off-by: Jiri Pirko <jiri@resnulli.us>