host: Remove sudo prompt from stderr in execute()

Remove the leading space introduced on stderr by: sudo -S -p ' '
background() still gets the space, since we cannot easily apply
processing to its stderr.

Note: -p '' does not work on recent sudo, so we unfortunately cannot
just completely remove it for the time being.

devlib/host.py

@@ -97,7 +97,8 @@ class LocalConnection(ConnectionBase):
     def execute(self, command, timeout=None, check_exit_code=True,
                 as_root=False, strip_colors=True, will_succeed=False):
         self.logger.debug(command)
-        if as_root and not self.connected_as_root:
+        use_sudo = as_root and not self.connected_as_root
+        if use_sudo:
             if self.unrooted:
                 raise TargetStableError('unrooted')
             password = self._get_password()
@@ -112,6 +113,11 @@ class LocalConnection(ConnectionBase):
                 raise TargetTransientError(message)
             else:
                 raise TargetStableError(message)
+
+        # Remove the one-character prompt of sudo -S -p
+        if use_sudo and stderr:
+            stderr = stderr[1:]
+
         return stdout + stderr
 
     def background(self, command, stdout=subprocess.PIPE, stderr=subprocess.PIPE, as_root=False):
@@ -119,6 +125,8 @@ class LocalConnection(ConnectionBase):
             if self.unrooted:
                 raise TargetStableError('unrooted')
             password = self._get_password()
+            # The sudo prompt will add a space on stderr, but we cannot filter
+            # it out here
             command = "echo {} | sudo -p ' ' -S ".format(quote(password)) + command
 
         # Make sure to get a new PGID so PopenBackgroundCommand() can kill