Fix FileStore.backup_file() not to backup same file

FileStore.backup_file() docstring claimed not to store a
copy of the same file but the behavior of the method did not
match this description.

This commit makes the backed-up file filename derivation
deterministic by hashing its content by SHA-256, thus it
should not back up two files with the same filename and content.

Reviewed-By: Aleksei Slaikovskii <aslaikov@redhat.com>

ipalib/install/sysrestore.py

@@ -29,6 +29,8 @@ import os.path
 import shutil
 import random
 
+from hashlib import sha256
+
 import six
 # pylint: disable=import-error
 if six.PY3:
@@ -111,7 +113,7 @@ class FileStore(object):
             p.write(f)
 
     def backup_file(self, path):
-        """Create a copy of the file at @path - so long as a copy
+        """Create a copy of the file at @path - as long as an exact copy
         does not already exist - which will be restored to its
         original location by restore_files().
         """
@@ -126,11 +128,11 @@ class FileStore(object):
 
         _reldir, backupfile = os.path.split(path)
 
-        filename = ""
-        for _i in range(8):
-            h = "%02x" % self.random.randint(0,255)
-            filename += h
-        filename += "-"+backupfile
+        with open(path, 'rb') as f:
+            cont_hash = sha256(f.read()).hexdigest()
+
+        filename = "{hexhash}-{bcppath}".format(
+                hexhash=cont_hash, bcppath=backupfile)
 
         backup_path = os.path.join(self._path, filename)
         if os.path.exists(backup_path):