Ensure that root_dir is an absolute path

If root_dir is not an absolute path `salt.utils.verify.check_parent_dirs` will return a problem, e.g with `root_dir: .`:

        ~/src/salt ] -> scripts/salt-master -l debug -c conf/master.template
        [DEBUG   ] Loaded master key: etc/salt/pki/master.pem
        [INFO    ] Preparing the root key for local communication
        [WARNING ] Starting the Salt Master
        [INFO    ] Starting the Salt Publisher on tcp://0.0.0.0:4505
        [INFO    ] Setting up the master communication server
        [INFO    ] Starting Salt worker process 0
        [INFO    ] Starting Salt worker process 1
        [INFO    ] Starting Salt worker process 2
        [INFO    ] Starting Salt worker process 3
        [INFO    ] Starting Salt worker process 4
        Process MWorker-6:
        Traceback (most recent call last):
          File "/usr/lib/python2.7/multiprocessing/process.py", line 258, in _bootstrap
            self.run()
          File "/home/uli/src/salt/salt/master.py", line 420, in run
            self.crypticle)
          File "/home/uli/src/salt/salt/master.py", line 959, in __init__
            self.local = salt.client.LocalClient(self.opts['conf_file'])
          File "/home/uli/src/salt/salt/client.py", line 78, in __init__
        Process MWorker-7:
        Traceback (most recent call last):
          File "/usr/lib/python2.7/multiprocessing/process.py", line 258, in _bootstrap
            self.key = self.__read_master_key()
          File "/home/uli/src/salt/salt/client.py", line 89, in __read_master_key
            salt.utils.verify.check_parent_dirs(keyfile, user)
          File "/home/uli/src/salt/salt/utils/verify.py", line 228, in check_parent_dirs
            self.run()
          File "/home/uli/src/salt/salt/master.py", line 420, in run
            raise SaltClientError(msg)
        SaltClientError: Could not access directory /cache. Please give uli read permissions.
            self.crypticle)
          File "/home/uli/src/salt/salt/master.py", line 959, in __init__
            self.local = salt.client.LocalClient(self.opts['conf_file'])
          File "/home/uli/src/salt/salt/client.py", line 78, in __init__
            self.key = self.__read_master_key()
          File "/home/uli/src/salt/salt/client.py", line 89, in __read_master_key
            salt.utils.verify.check_parent_dirs(keyfile, user)
          File "/home/uli/src/salt/salt/utils/verify.py", line 228, in check_parent_dirs
            raise SaltClientError(msg)
        SaltClientError: Could not access directory /cache. Please give uli read permissions.
        Process MWorker-5:
        Traceback (most recent call last):
          File "/usr/lib/python2.7/multiprocessing/process.py", line 258, in _bootstrap
            self.run()
          File "/home/uli/src/salt/salt/master.py", line 420, in run
            self.crypticle)
          File "/home/uli/src/salt/salt/master.py", line 959, in __init__
            self.local = salt.client.LocalClient(self.opts['conf_file'])
          File "/home/uli/src/salt/salt/client.py", line 78, in __init__
            self.key = self.__read_master_key()
          File "/home/uli/src/salt/salt/client.py", line 89, in __read_master_key
            salt.utils.verify.check_parent_dirs(keyfile, user)
          File "/home/uli/src/salt/salt/utils/verify.py", line 228, in check_parent_dirs
            raise SaltClientError(msg)
        SaltClientError: Could not access directory /cache. Please give uli read permissions.
        Process MWorker-8:
        Traceback (most recent call last):
          File "/usr/lib/python2.7/multiprocessing/process.py", line 258, in _bootstrap
            self.run()
          File "/home/uli/src/salt/salt/master.py", line 420, in run
            self.crypticle)
          File "/home/uli/src/salt/salt/master.py", line 959, in __init__
            self.local = salt.client.LocalClient(self.opts['conf_file'])
          File "/home/uli/src/salt/salt/client.py", line 78, in __init__
            self.key = self.__read_master_key()
          File "/home/uli/src/salt/salt/client.py", line 89, in __read_master_key
            salt.utils.verify.check_parent_dirs(keyfile, user)
          File "/home/uli/src/salt/salt/utils/verify.py", line 228, in check_parent_dirs
            raise SaltClientError(msg)
        SaltClientError: Could not access directory /cache. Please give uli read permissions.
        ^C[WARNING ] Stopping the Salt Master

This commit fixes that problem by calling `os.path.abspath` before using `root_dir`