ipmi_adapter: simplify, retry if command fails

ipmi_adapter: simplify, retry if command fails

The method get_node_state has been added to the the IpmiAdapter class.

In addition, now the power on/off methods will try several times to
perform their IPMI command before giving up, instead of bailing out at
the first error.

After the power on/off command is completed, the method will wait until
the node is in the desired state.

NOTE: a command could potentially take several minutes if the defaults
are used; each IPMI command can take up to 1 minute, and there can be 3
commands issued per operation, one of them may be retried 20 times with
the current defaults. Ideally we would use eventlet or something similar
to allow each command a limited time to execute, instead:

    with eventlet.timeout.Timeout(seconds) as t:
        power_on/off_command

Signed-off-by: Josep Puigdemont <josep.puigdemont@enea.com>