Use extension constants rather than static strings

Today kuryr-libnetwork appears to reference (and thus depend on) some neutron extension values.
For example kuryr_libnetwork.controllers references the tag and tag-ext ALIAS as follows today:

TAG_NEUTRON_EXTENSION = "tag"
TAG_EXT_NEUTRON_EXTENSION = "tag-ext"

While this works, a more complete approach is to use the ALIAS attribute from the extension (or API definition if already in neutron-lib). This not only removes the need to use redundant static strings, but is also declares intent to use the extension. The later is important as when we consume neutron-lib changes we search for imports.

This is the main reason for the issue in [1]; because the project didn't import the extensions it was not addressed when making the changes in neutron.

Please evaluate the code in the kuryr-libnetwork project to ensure extensions and constant values are properly referenced from neutron and/or neutron-lib.

Thanks

[1] https://review.openstack.org/#/c/544179