Avoid repeating default context with literal text

Signed-off-by: Chenxiong Qi <cqi@redhat.com>

module_build_service/models.py

@@ -40,6 +40,8 @@ import module_build_service.messaging
 from sqlalchemy.orm import lazyload
 from sqlalchemy import func, and_
 
+DEFAULT_MODULE_CONTEXT = '00000000'
+
 
 # Just like koji.BUILD_STATES, except our own codes for modules.
 BUILD_STATES = {
@@ -173,7 +175,7 @@ class ModuleBuild(MBSBase):
     ref_build_context = db.Column(db.String)
     build_context = db.Column(db.String)
     runtime_context = db.Column(db.String)
-    context = db.Column(db.String, server_default='00000000')
+    context = db.Column(db.String, server_default=DEFAULT_MODULE_CONTEXT)
     state = db.Column(db.Integer, nullable=False)
     state_reason = db.Column(db.String)
     modulemd = db.Column(db.String, nullable=False)

module_build_service/resolver/DBResolver.py

@@ -139,7 +139,7 @@ class DBResolver(GenericResolver):
                 queried_mmd = mmd
                 nsvc = ':'.join([
                     mmd.get_name(), mmd.get_stream(), str(mmd.get_version()),
-                    mmd.get_context() or '00000000'])
+                    mmd.get_context() or models.DEFAULT_MODULE_CONTEXT])
             else:
                 build = models.ModuleBuild.get_build_from_nsvc(
                     session, name, stream, version, context)

module_build_service/resolver/MBSResolver.py

@@ -245,7 +245,7 @@ class MBSResolver(GenericResolver):
                 continue
 
             if "context" not in details:
-                details["context"] = "00000000"
+                details["context"] = models.DEFAULT_MODULE_CONTEXT
             modules = self._get_modules(
                 name, details['stream'], details['version'],
                 details['context'], strict=True)

module_build_service/utils/general.py

@@ -264,7 +264,7 @@ def import_mmd(session, mmd):
              log messages collected during import (list)
     :rtype: tuple
     """
-    mmd.set_context("00000000")
+    mmd.set_context(models.DEFAULT_MODULE_CONTEXT)
     name = mmd.get_name()
     stream = mmd.get_stream()
     version = str(mmd.get_version())