empanadas: Eliminate user agent template hack

Jinja's official method for including template-like strings within
templates is to use variable blocks with quoted strings inside.  We will
eliminate the need for callers to provide something which the mock
template can provide on its own.

Author: trinity-q

iso/empanadas/empanadas/templates/isomock.tmpl.cfg

@@ -35,7 +35,7 @@ best=1
 install_weak_deps=0
 protected_packages=
 module_platform_id=platform:{{ dist }}
-user_agent={{ user_agent }}
+user_agent={{ '{{' }} user_agent {{ '}}' }}
 
 {% for repo in repos %}
 [{{ repo.name }}]

iso/empanadas/empanadas/util/iso_utils.py

@@ -229,7 +229,6 @@ def generate_iso_scripts(self):
                 required_pkgs=required_pkgs,
                 dist=self.disttag,
                 repos=self.repolist,
-                user_agent='{{ user_agent }}',
                 compose_dir_is_here=self.compose_dir_is_here,
                 compose_dir=self.compose_root,
         )
@@ -802,7 +801,6 @@ def _extra_iso_local_config(self, arch, image, grafts, work_root):
                 required_pkgs=required_pkgs,
                 dist=self.disttag,
                 repos=self.repolist,
-                user_agent='{{ user_agent }}',
                 compose_dir_is_here=True,
                 compose_dir=self.compose_root,
         )
@@ -1691,7 +1689,6 @@ def _live_iso_local_config(self, image, work_root):
                 dist=self.disttag,
                 repos=self.repolist,
                 compose_dir_is_here=True,
-                user_agent='{{ user_agent }}',
                 compose_dir=self.compose_root,
         )