remove slash from kubernetes cluster context name
remove slash from kubernetes cluster context name

magnum names kubernetes clusters context as "default/something",
whose slash may cause some problems with certains tools, such as
kubefed. remove the slash from the context name.

Change-Id: I9b0c64b524d303ed51d5d2f3e0464245bcc66896
Closes-Bug: #1705480

openstack/python-magnumclient