Add support for version-locked in buildsys-tags

We are in the proccess of adding a bot to bump buildsys-tags
in rdoinfo (see [1]). This adds an option to lock packages
based on a new buildsys-tag version-locked as done in tags.

This patch adds support for this new tag in the verify.py script
used to validate files syntax.

[1] https://review.rdoproject.org/r/#/c/7360/

Change-Id: I27a23c19ea1e6fc07786eca7a0f4d631885cbc29

README.rst

@@ -82,6 +82,11 @@ a project:
   tag in CentOS Build System (CBS). It contains a set of mappings where the key is the
   CBS tag name and the value is the package NVR included in the tag.
 
+  * *version-locked* can be used as buildsys-tag key to disable automatic update by
+    periodic job. if a list of CBS tags is assigned to version-locked buildsys-tag,
+    only the tags in the list are locked. If it's empty automatic job will ignore the
+    package for all tags.
+
 
 Attributes inheritance in rdoinfo
 ---------------------------------

verify.py

@@ -18,7 +18,7 @@ def verify_buildsys_tags(pkg, buildsystags):
         for btag in btags.keys():
             if btag in buildsystags:
                 value = btags[btag]
-                if value is None:
+                if value is None and btag != 'version-locked':
                     raise Exception("buildsys-tag %s for package %s is empty" %
                                     (btag, pkg['name']))
             else:
@@ -27,7 +27,7 @@ def verify_buildsys_tags(pkg, buildsystags):
     return True
 
 def list_buildsys_tags(info):
-    tags = []
+    tags = ['version-locked']
     for release in info['releases']:
         for repo in release['repos']:
             if 'buildsys-tags' in repo.keys():