CLOUDSTACK-5707. Hitting multiple 'User unauthenticated' exceptions when vCenter is upgraded to 5.5 from 5.1.
We hit these excptions whenever a management server held session that was with the old 5.1 vCenter server
is used to make resource calls to the new 5.5 vCenter.
Validate a vCenter session context before it is being used to make a resource call.
And if the context is invalid then discard the context and retrieve a new one.
During the invalidation of an old context handle the context disconnect better
by catching the appropriate exception and returning a newly created context.