stbt-power: Use the HTTP interface of PDUeX units

The PDUeX KWX device provides three interfaces:
- SSH: The one used so far; very slow (it takes up to 20 seconds to
  execute a command) and we are experiencing PDU lock-ups during high
  load.
- Telnet: It's fast, but doesn't accept multiple simultaneous client
  connections.
- HTTP: Fast and handles concurrency.

Considering the above mentioned pros and cons, it's worth switching
to HTTP.

This commit changes the output format:
- 'on' and 'off' commands don't output anything unless there is an error;
- 'status' only prints "ON" or "OFF" if the individual power outlet is
  specified;
- or "<outlet>: ON|OFF" with the status of one outlet per output line if
  all outlets are queried.

IMPLEMENTATION ISSUES

1. FAILED COMMANDS

Stress-tested the PDU with 8 simultaneous `stbt power` commands sent every
~10 seconds for 64 hours. The device did not lock up, but out of ~300000
on/off requests 104 failed, that is, ~0.04 % of all requests.

To minimise the number of failures, every 'on' and 'off' request is sent
to the PDU up to three times, allowing 2 ignored requests per `stbt power`
execution. If the 3rd request fails, `stbt power` exits with status code 1.

2. FAILED AUTHENTICATION

The PDU uses basic access authentication. The steps are the following:
i.   The client sends a request.
ii.  The server rejects it with 401: Unauthorized and a WWW-Authenticate
     header.
iii. The client resends the request with an Authorization header.
iv.  The server sends the expected response.

Occasionally, when the PDU's HTTP server receives multiple simultaneous
requests, it rejects the resent request (iii.) and responds with 401
one more time. To minimise the number of such failures, the authentication
process is repeated if the first attempt fails.

Points 1. and 2. reduce the failure rate to virtually zero.

3. TIMING

Measuring execution time showed that the PDU requires up to 1
second per outlet to execute a command, i.e. the status of the
outlet changes within 1 second if 1 command is sent, but it may
take 8 seconds if 8 simultaneous commands are sent.

As the PDU doesn't send response to 'on'/'off' requests, the status of
the outlet is queried to verify the outcome of a command. To save time,
status requests are sent every second up to 8 times; if the 8th
response doesn't include the expected status, the 'on'/'off' request is
resent.

4. HARD-CODED IDENTIFIERS

It is assumed that an individual outlet address is always in the format
'1-An' where 'n' is the number of the individual power outlet in the
range 1 <= n <= 8. We need to have this assumption because 'on' and 'off'
HTTP requests don't include the outlet address, only the outlet number;
however we need to know the outlet address to parse the status response.

It is also assumed that the default user name and password 'admin:admin'
haven't been changed on the PDU unit.