OpenStack: add functionality to attach FloatingIP to Port

[JeanMertz]

A couple of notes:

• I didn't write any tests, because I apparently can't even break any tests. Whenever I change random test values in _test.go files, and run go test ./..., all tests still pass. As soon as I manage to get a test-workflow going, I will try my hand at the tests.
• I tested this on my own OpenStack environment, and things work as expected.
• The possibility to actually manage ports is introduced here: implement "openstack_networking_port_v2" resource #1789

[mitchellh]

LGTM, same with tests here

[josephholsten]

@mitchellh is this waiting on anything to merge?

[josephholsten]

@phinze this seems ready to merge

[DanyC97]

+1

[phinze]

LGTM - thank you!

[ghost]

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