azure backend not working with latest Azure Storage SDK for Python (lp:#1694770)

[Original report](https://bugs.launchpad.net/bugs/1694770) created by **Rafal Sosinski (rafal.sosinski)**
```
There is a problem using latest version of duplicity (0.7.12) with latest
Azure Storage SDK for Python (2.0.0) for access Azure resources

root@restore:~# duplicity --file-to-restore data/Capability --no-encryption
azure://backup05162017 /mnt -v9
Using archive dir: /root/.cache/duplicity/851a65c225c23ac6257e3fb3ed9061aa
Using backup name: 851a65c225c23ac6257e3fb3ed9061aa
GPG binary is gpg, version 1.4.20
Import of duplicity.backends.acdclibackend Succeeded
Import of duplicity.backends.azurebackend Succeeded
Import of duplicity.backends.b2backend Succeeded
Import of duplicity.backends.botobackend Succeeded
Import of duplicity.backends.cfbackend Succeeded
Import of duplicity.backends.copycombackend Succeeded
Import of duplicity.backends.dpbxbackend Failed: No module named dropbox
Import of duplicity.backends.gdocsbackend Succeeded
Import of duplicity.backends.giobackend Succeeded
Import of duplicity.backends.hsibackend Succeeded
Import of duplicity.backends.hubicbackend Succeeded
Import of duplicity.backends.imapbackend Succeeded
Import of duplicity.backends.lftpbackend Succeeded
Import of duplicity.backends.localbackend Succeeded
Import of duplicity.backends.mediafirebackend Succeeded
Import of duplicity.backends.megabackend Succeeded
Import of duplicity.backends.multibackend Succeeded
Import of duplicity.backends.ncftpbackend Succeeded
Import of duplicity.backends.onedrivebackend Succeeded
Import of duplicity.backends.par2backend Succeeded
Import of duplicity.backends.pydrivebackend Succeeded
Import of duplicity.backends.rsyncbackend Succeeded
Import of duplicity.backends.ssh_paramiko_backend Succeeded
Import of duplicity.backends.ssh_pexpect_backend Succeeded
Import of duplicity.backends.swiftbackend Succeeded
Import of duplicity.backends.sxbackend Succeeded
Import of duplicity.backends.tahoebackend Succeeded
Import of duplicity.backends.webdavbackend Succeeded
Using temporary directory /tmp/duplicity-omCmzF-tempdir
Backend error detail: Traceback (most recent call last):
  File "/usr/bin/duplicity", line 1546, in <module>
    with_tempdir(main)
  File "/usr/bin/duplicity", line 1540, in with_tempdir
    fn()
  File "/usr/bin/duplicity", line 1375, in main
    action = commandline.ProcessCommandLine(sys.argv[1:])
  File "/usr/lib/python2.7/dist-packages/duplicity/commandline.py", line
1140, in ProcessCommandLine
    backup, local_pathname = set_backend(args[0], args[1])
  File "/usr/lib/python2.7/dist-packages/duplicity/commandline.py", line
1015, in set_backend
    globals.backend = backend.get_backend(bend)
  File "/usr/lib/python2.7/dist-packages/duplicity/backend.py", line 223,
in get_backend
    obj = get_backend_object(url_string)
  File "/usr/lib/python2.7/dist-packages/duplicity/backend.py", line 209,
in get_backend_object
    return factory(pu)
  File "/usr/lib/python2.7/dist-
packages/duplicity/backends/azurebackend.py", line 53, in __init__
    Exception: %s""" % str(e))
BackendException: Azure backend requires Microsoft Azure Storage SDK for
Python (https://pypi.python.org/pypi/azure-storage/).
Exception: cannot import name BlobService


Required package is already installed

root@restore:~# pip freeze
adal==0.4.5
asn1crypto==0.22.0
azure-common==1.1.6
azure-nspkg==2.0.0
azure-storage==0.34.2
certifi==2017.4.17
cffi==1.10.0
cryptography==1.8.1
duplicity===-version
enum34==1.1.6
futures==3.1.1
idna==2.5
ipaddress==1.0.18
isodate==0.5.4
keyring==10.3.2
lockfile==0.12.2
msrest==0.4.8
oauthlib==2.0.2
packaging==16.8
pathlib2==2.2.1
pexpect==4.0.1
ptyprocess==0.5
pycparser==2.17
pycrypto==2.6.1
PyJWT==1.5.0
pyparsing==2.2.0
python-dateutil==2.6.0
requests==2.14.2
requests-oauthlib==0.8.0
scandir==1.5
SecretStorage==2.3.1
six==1.10.0

There is no error when using old version of azure package (0.11.1)

Problem is on Linux (Debian, Ubuntu, RHEL and CentOS)
```

Story points: 34.0