Add generic re-try logic to all of fedimg's operations

[ralphbean]

Right now, the cloud just plain fails sometimes. Like this error email I got today:

[2015-09-17 13:20:12][    fedmsg   ERROR]
Image copy to ap-southeast-2 failed


Process Details
---------------
host:     fedimg01.phx2.fedoraproject.org
PID:       8640
name:     fedmsg-hub
command:  /usr/bin/python /usr/bin/fedmsg-hub

Callstack that lead to the logging statement
--------------------------------------------
  File "/usr/lib64/python2.7/threading.py", line 784 in __bootstrap
    self.__bootstrap_inner()
  File "/usr/lib64/python2.7/threading.py", line 811 in __bootstrap_inner
    self.run()
  File "/usr/lib64/python2.7/threading.py", line 764 in run
    self.__target(*self.__args, **self.__kwargs)
  File "/usr/lib64/python2.7/multiprocessing/pool.py", line 113 in worker
    result = (True, func(*args, **kwds))
  File "/usr/lib64/python2.7/multiprocessing/pool.py", line 65 in mapstar
    return map(*args)
  File "/usr/lib/python2.7/site-packages/fedimg/uploader.py", line 46 in <lambda>
    results = pool.map(lambda s: s.upload(), services)
  File "/usr/lib/python2.7/site-packages/fedimg/services/ec2.py", line 627 in upload
    ami['region']))
Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/fedimg/services/ec2.py", line 605, in upload
    description=self.image_desc)
  File "/usr/lib/python2.7/site-packages/libcloud/compute/drivers/ec2.py", line 2691, in copy_image
    self.connection.request(self.path, params=params).object)
  File "/usr/lib/python2.7/site-packages/libcloud/common/base.py", line 799, in request
    response = responseCls(**kwargs)
  File "/usr/lib/python2.7/site-packages/libcloud/common/base.py", line 143, in __init__
    headers=self.headers)
BaseHTTPError: InternalError: An internal error has occurred

The cloud just doesn't always work 100% of the time; we know that.

So, we should wrap most of fedimg's operations in little re-try loops that try 3 or 4 times before giving up.