fix: Allow running on systems without /etc/fstab present

When /etc/fstab is not present, we'll simply skip parsing it.

Fixes: #557
Resolves: RHEL-115033

Author: vojtechtrefny

library/blivet.py

@@ -2186,6 +2186,9 @@ def parse(self):
         if self._entries:
             self.reset()
 
+        if not os.path.exists('/etc/fstab'):
+            return
+
         with open('/etc/fstab') as f:
             for line in f.readlines():
                 if line.lstrip().startswith("#"):