Move `_verifySignatureInCertificateList` to a public method

Note: Forgotten commit.

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)

As we can see today:

AssertionError: ' 2119 ' not found in 'notAfter=Dec 14 00:54:31 2118 GMT\n')

Since OpenSSL 1.1 those broken certificates became a problem during testing,
so regenerate them with new OpenSSL.

/reviewed-on !2

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

Please, @kazuhiko and @kirr, review this.

See merge request !1

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.