Prefer security file over username/password

[thotypous]

Currently, it is impossible to use the VNX driver in cinder (master branch) with a security file, whereas the old driver (for stable/mitaka) used to work well.

The cause of the issue is as follows

• Storops side:
  • username and password currently take precedence over sec_file if both are defined (defined meaning different from None).
• Cinder side:
  • san_login and san_password have default values ('admin' and '', respectively): cinder/volume/drivers/san/san.py:45
  • storage_vnx_security_file_dir does not have a default value: cinder/volume/drivers/emc/vnx/common.py:46
  • It is impossible to override san_login and san_password in such a way to get None as their value.

The proposed fix inverts the precedence in storops. This way, the following configuration in cinder.conf works fine:

[vnx]
storage_vnx_pool_names=Cloud_Pool
volume_backend_name=vnx
naviseccli_path=/opt/Navisphere/bin/naviseccli
volume_driver=cinder.volume.drivers.emc.vnx.driver.EMCVNXDriver
storage_vnx_security_file_dir=/var/lib/cinder
initiator_auto_registration=True
san_ip=172.16.0.102
storage_protocol=iscsi

Another possible fix would be to consider empty strings as undefined (i.e. change if self._username is None and self._password is None to if not self._username and not self._password), but this would make Mitaka's cinder.conf even more incompatible with Newton's one, as users would be required to declare san_login as an empty string.

Otherwise, things could be fixed at the cinder side, but it feels weird to convert an empty string to None before passing it to a library. On the other hand, removing default values for san_login and san_password could have implications for users of other storage backends.

[coveralls]

Coverage decreased (-0.02%) to 92.271% when pulling ef37614 on thotypous:bug/cinder-secfile into 8ea8c5a on emc-openstack:master.

[coveralls]

Coverage remained the same at 92.294% when pulling 3c4cd91 on thotypous:bug/cinder-secfile into 8ea8c5a on emc-openstack:master.

[coveralls]

Coverage remained the same at 92.294% when pulling 6fc4425 on thotypous:bug/cinder-secfile into 8ea8c5a on emc-openstack:master.

[jealous]

Hi @thotypous , thanks for the contribution. This change looks fine to me.
Could you do two more things?

1. Add your name to the contributor list in README.md.
2. Pop the version to 0.2.19 in README.md so that I could push a new version in pypi.

[thotypous]

Hi @jealous , thank you for the review! I just updated the patch including the changes to README.md.

[coveralls]

Coverage decreased (-0.02%) to 92.271% when pulling 87904a5 on thotypous:bug/cinder-secfile into 8ea8c5a on emc-openstack:master.

[coveralls]

Coverage decreased (-0.02%) to 92.271% when pulling 04c622f on thotypous:bug/cinder-secfile into 8ea8c5a on emc-openstack:master.