Append loaded_certificate in the try block, so it does not result with
"UnboundLocalError: local variable 'loaded_certificate' referenced before assignment"

python 3 compatibility

When list of signature certificates is processed any of them can result
with not using a cache while download, which is bad situation.

Just ignore bad certificates while checking the list, so any other good
ones can be used.

/reviewed-on !4

"context.set_ciphers('DEFAULT:@SECLEVEL=1')" was introduced in
ab3b2c3b but for very old SSL we can have the
follwoing error:

Traceback (most recent call last):
  File "/usr/local/bin/networkcache-upload", line 9, in <module>
    load_entry_point('slapos.libnetworkcache==0.19', 'console_scripts', 'networkcache-upload')()
  File "/usr/local/lib/python2.7/dist-packages/slapos/libnetworkcache.py", line 489, in cmd_upload
    **dict(x.split('=', 1) for x in args.meta))
  File "/usr/local/lib/python2.7/dist-packages/slapos/libnetworkcache.py", line 341, in upload
    'Content-Type': 'application/octet-stream'})
  File "/usr/local/lib/python2.7/dist-packages/slapos/libnetworkcache.py", line 264, in _request
    context.set_ciphers('DEFAULT:@SECLEVEL=1') # XXX
ssl.SSLError: ('No cipher can be selected.',)

/reviewed-on !3

Add download auto retrying function. This is necessary because downloading binary cache often fails.

Traceback (most recent call last):
  ...
  File "/usr/lib/python3.7/http/client.py", line 1376, in __init__
    context.load_cert_chain(cert_file, key_file)
ssl.SSLError: [SSL: CA_MD_TOO_WEAK] ca md too weak (_ssl.c:3824)

The commands remain difficult to use. Maybe we should move some slapos.buildout
code here so that the required information can be computed automatically.

Ensure that every string key and value of the json response from the network cache server is a str object.

This method was added mostly because the NetworkcacheClient gets a json response from the server and all textual keys and values after json.loads are unicode objects, which may result in a error later when we try to join the url and the parameters of the package to use the networkcache.

We can't depend on pyOpenSSL because SlapOS is not ready for that.

To simplify things, DirectoryNotFound & UploadError are merged in
NetworkcacheException.

This also fixes a bug returning wrong metadata when several entries exists
in shadirs.

- Deprecate slapos.networkcachehelper
- Common internal method to query network cache, to reduce code duplication
  and fix downloading though SSL.
- Do not upload data that is already in SHACACHE. This reverts commit
  7bb5e112 partially, since it's not possible
  to hash on the fly. 'tempfile' module is reimported for non-seekable streams.
- New way to instanciate NetworkcacheClient, again to reduce code duplication,
  but also to use an instance for both upload & download.
  Old way is still there for compatibility until it is unused.

If people don't write stupid code like 'if result is True:',
this small API change should not break anything.

Add new index() method to add an entry to shadir when the data is known to be
already in shacache.

When uploading big file, this avoids trashing the kernel cache once more.