gitignore: add .eggs

When running tox, it creates the .eggs folder and leaves the repo
reporting that it's in a non-clean state.

Change-Id: If09fda47af1be8bd34434d82e31a4b7fb7d86e83

.gitignore

@@ -1,5 +1,6 @@
 .tox
 doc/build
+.eggs
 *.egg-info
 pbr*.egg
 reference/projects/*.rst
@@ -9,4 +10,4 @@ ChangeLog
 .stestr
 .coverage.*
 .coverage
-cover/
+cover/