util/mnt: add explicit rw option

Under certain conditions a bind mount without a specified `rw` or `ro`
option gets mounted read-only.  We need a way to be explicit about
needing a rw mount.  We might want to change this in the future to be a
single option (mode optional?) with valid values "rw", "ro".

It's not entirely clear what the conditions are but it occurs when bind
mounting the containers storage into the osbuild store, which we will
need for the next few commits.

osbuild/util/mnt.py

@@ -44,12 +44,14 @@ class MountGuard(contextlib.AbstractContextManager):
     def __init__(self):
         self.mounts = []
 
-    def mount(self, source, target, bind=True, ro=False, mode="0755"):
+    def mount(self, source, target, bind=True, ro=False, rw=False, mode="0755"):
         options = []
         if bind:
             options += ["bind"]
         if ro:
             options += ["ro"]
+        if rw:
+            options += ["rw"]
         if mode:
             options += [mode]