Limit the variants with config option 'tree_variants'

'tree_variants' in configuration should be able to limit the tree
variants, just like the config option 'tree_arches' which is used to
limited the arches.

For example, if there is configuration (refer to doc/configuration.rst)
go with:

        tree_variants = ["Server"]

then only "Server" variants will be composed, and any other variants in
variants.xml will be ignored.

Signed-off-by: Qixiang Wan <qwan@redhat.com>

Rebased on master

Signed-off-by: Dennis Gilmore <dennis@ausil.us>

doc/configuration.rst

@@ -113,6 +113,12 @@ Example
     base_product_short = "Fedora"
     base_product_version = "23"
 
+**tree_arches**
+    ([*str*]) -- list of architectures which should be included; if undefined, all architectures from variants.xml will be included
+
+**tree_variants**
+    ([*str*]) -- list of variants which should be included; if undefined, all variants from variants.xml will be included
+
 
 General Settings
 ================
@@ -144,6 +150,8 @@ Options
 
     Please note that ``*`` as a wildcard matches all architectures but ``src``.
 
+    tree_arches = ["x86_64"]
+    tree_variants = ["Server"]
 
 Example
 -------

pungi/compose.py

@@ -215,7 +215,10 @@ class Compose(kobo.log.LoggingBase):
     def get_variants(self, types=None, arch=None, recursive=False):
         result = []
         types = types or ["variant", "optional", "addon", "layered-product"]
+        tree_variants = self.conf.get("tree_variants", None)
         for i in self.variants.values():
+            if tree_variants and i.name not in tree_variants:
+                continue
             if i.type in types:
                 if arch and arch not in i.arches:
                     continue