session_persistence: invalid str2dict value: u''

Can't receive empty session_persistence dict to update the vip.
It would raise the following error:

$ neutron lb-vip-update vip --session_persistence type=dict ""
usage: neutron [--session_persistence SESSION_PERSISTENCE]
neutron: error: argument --session_persistence: invalid str2dict value: u''

Change-Id: Ice43bbe19138be6af85b47c2090359d22c70b29f
Closes-Bug: #1262582
Suppress stdout/stderr in test_shell

test_shell is refactored to check stderr in addition to stdout
and suppress stdout/stderr. In addition, test_auth tries API call
to keystone, so related methods are mocked out now to avoid it.

Change-Id: Id43ad905c5c13da9015d035267eaa86429e60ff3
Closes-Bug: #1161849
Merge "Suppress stdout/stderr in test_shell"
Merge "session_persistence: invalid str2dict value: u''"