New command entry for acloud pull function

- Add new command for '$acloud pull'.
- Add args "--instance-name" to specify which instance to pull.

Bug: 120613398
Bug: 112274919
Test: acloud-dev pull
      acloud-dev pull --instance-name ins_name

Change-Id: I45e43a2a2a63dec3bae475fff57fde398e4cc915
8 files changed