nova: support to run script automatically with AMT

This enables us to use the run scripts applied to a native machine equipped
with Intel's AMT. If the environment variables are correctly set up, the remote
test machine is reseted via 'amttool', then via 'amtterm' the serial output
is collected and the normal run script matching pattern for success/failure of
the run script are applied.

'amttool' and 'amtterm' are part of the package called 'amtterm' shipped with
the Linux distributions like Ubuntu, Debian and lot more.

Following environment variables are required, to run the run scripts with a
native AMT test machine:

PXE_TFTP_DIR_BASE   - absolute path of TFTP directory
PXE_TFTP_DIR_OFFSET - relative path to PXE_TFTP_DIR_BASE where the config file
                      will be generated - named 'config-00-00-00-00-00-00'

AMT_TEST_MACHINE_IP  - TCP/IP address of target AMT test machine
AMT_TEST_MACHINE_PWD - password of target AMT test machine

Issue #679