[HTTP] Allow curl to retry on transient errors

[segiddins]

From the curl docs:

          If a transient error is returned when curl tries  to  perform  a
          transfer,  it  will retry this number of times before giving up.
          Setting the number to 0 makes curl do no retries (which  is  the
          default).  Transient  error  means either: a timeout, an FTP 4xx
          response code or an HTTP 5xx response code.

          When curl is about to retry a transfer, it will first  wait  one
          second  and  then for all forthcoming retries it will double the
          waiting time until it reaches 10 minutes which then will be  the
          delay  between  the rest of the retries.  By using --retry-delay
          you  disable  this  exponential  backoff  algorithm.  See   also
          --retry-max-time to limit the total time allowed for retries.

          If this option is used several times, the last one will be used.

          Added in 7.12.3.