Check for mock version instead of redhat_release

redhat_release is what distro we're _building for_ which is not a good test
for which version of mock is installed.

osgbuild/mock.py

@@ -37,6 +37,13 @@ def __init__(self, buildopts, koji_obj):
 
         cfg_path = self._init_get_cfg_path()
         self.mock_cmd = ['mock']
+        mock_version_str = utils.backtick(self.mock_cmd + ["--version"]).strip()
+        m = re.match(r"(\d+)\.(\d+)\.(\d+)", mock_version_str)
+        if m:
+            self.mock_version = (int(m.group(1)), int(m.group(2)), int(m.group(3)))
+        else:
+            raise MockError("mock --version returned unexpected output: %s" % mock_version_str)
+
         if cfg_path:
             cfg_abspath = os.path.abspath(cfg_path)
             cfg_abspath_no_ext = re.sub(r'\.cfg$', '', cfg_abspath)
@@ -133,13 +140,12 @@ def rebuild(self, resultdir, srpm):
                                        srpm]
         if self.target_arch:
             rebuild_cmd += ['--arch', self.target_arch]
-        redhat_release = int(self.buildopts['redhat_release'])
-        if redhat_release == 6:
-            # ccache on el6 tries to install the el5 package for ccache and dies
-            rebuild_cmd += ['--disable-plugin=ccache']
-        elif redhat_release >= 7:
+        if self.mock_version >= (1,4,0):
             # systemd-nspawn is often broken; don't use it. network is required for maven builds :(
             rebuild_cmd += ['--enable-network', '--config-opts=use_nspawn=False']
+        else:
+            # ccache on old versions tries to install the el5 package for ccache and dies
+            rebuild_cmd += ['--disable-plugin=ccache']
         ret = utils.unchecked_call(rebuild_cmd)
         if ret:
             raise MockError('Mock build failed (command was: ' + ' '.join(rebuild_cmd) + ')')
@@ -149,7 +155,6 @@ def rebuild(self, resultdir, srpm):
         return rpms
 
 
-
     def clean(self):
         """Clean the mock chroot"""
         utils.checked_call(self.mock_cmd + ["clean"])