Add support for including files using the default template processor

The syntax is '{% include "file" %}' where file is either an absolute path or a
path relative to the current template file's directory.

Variables in the included file will be replaced as for the main template. But
the included file can't include files itself.

test/test_unit_template_default.py

@@ -88,6 +88,36 @@ Included section for distro = {LOCAL_DISTRO} ({LOCAL_DISTRO} again)
 end of template
 '''
 
+INCLUDE_BASIC = 'basic\n'
+INCLUDE_VARIABLES = f'''\
+included <{{{{ yadm.class }}}}> file
+
+empty line above
+'''
+INCLUDE_NESTED = 'no newline at the end'
+
+TEMPLATE_INCLUDE = '''\
+The first line
+{% include empty %}
+An empty file removes the line above
+{%include basic%}
+{% include "./variables.{{ yadm.os }}"  %}
+{% include dir/nested %}
+Include basic again:
+{% include basic %}
+'''
+EXPECTED_INCLUDE = f'''\
+The first line
+An empty file removes the line above
+basic
+included <{LOCAL_CLASS}> file
+
+empty line above
+no newline at the end
+Include basic again:
+basic
+'''
+
 
 def test_template_default(runner, yadm, tmpdir):
     """Test template_default"""
@@ -132,3 +162,37 @@ def test_source(runner, yadm, tmpdir):
     assert run.err == ''
     assert output_file.read().strip() == str(input_file)
     assert os.stat(output_file).st_mode == os.stat(input_file).st_mode
+
+
+def test_include(runner, yadm, tmpdir):
+    """Test include"""
+
+    empty_file = tmpdir.join('empty')
+    empty_file.write('', ensure=True)
+
+    basic_file = tmpdir.join('basic')
+    basic_file.write(INCLUDE_BASIC)
+
+    variables_file = tmpdir.join(f'variables.{LOCAL_SYSTEM}')
+    variables_file.write(INCLUDE_VARIABLES)
+
+    nested_file = tmpdir.join('dir').join('nested')
+    nested_file.write(INCLUDE_NESTED, ensure=True)
+
+    input_file = tmpdir.join('input')
+    input_file.write(TEMPLATE_INCLUDE)
+    input_file.chmod(FILE_MODE)
+    output_file = tmpdir.join('output')
+
+    script = f"""
+        YADM_TEST=1 source {yadm}
+        set_awk
+        local_class="{LOCAL_CLASS}"
+        local_system="{LOCAL_SYSTEM}"
+        template_default "{input_file}" "{output_file}"
+    """
+    run = runner(command=['bash'], inp=script)
+    assert run.success
+    assert run.err == ''
+    assert output_file.read() == EXPECTED_INCLUDE
+    assert os.stat(output_file).st_mode == os.stat(input_file).st_mode