Split test_rebuild() into two tests

The method tests rebuilds with and without passwords, but the tests are
independent and should be in two seperate tests.

Also, remove duplication of the rebuild with password test from
test_rebuild_preserve_ephemeral

Change-Id: I40d0376fabf319dbe703d593de8fb4cdcf0aeab1
Remove py3kcompat

It's not used by novaclient anymore and isn't used by any other
openstack common code either.

Change-Id: I96ebf030d0a96436a8a75c6937c354003ee32c58
Closes-bug: 1280033
Print adminPass when rebuilding from shell

The 'nova rebuild' command did not print the adminPass returned by the
server. This is because it discards the instance information returned
by the rebuild action and fetchs the information again. This would lose
the adminPass and it would not be printed for the user.

This patch reuses the instance information returned by the rebuild action
which ensures that the adminPass is printed by the shell.

Change-Id: I144bfa298bef529c78ca04042cd37a763537f09e
Closes-bug: 1302696
Fix session handling in novaclient

Prior to this patch, novaclient was handling sessions in an inconsistent
manner.

Every time we created a client instance, it would use a global
connection pool, which made it difficult to use in a process that is
meant to be forked.

Obviously sessions like the ones provided by the requests library that
will automatically cause connections to be kept alive should not be
implicit. This patch moves the novaclient back to the age of a single
session-less request call by default, but also adds two more
resource-reuse friendly options that a user needs to be explicit about.

The first one is that both v1_1 and v3 clients can now be used as
context managers,. where the session will be kept open (and thus the
connection kept-alive) for the duration of the with block. This is far
more ideal for a web worker use-case as the session can be made
request-long.

The second one is the per-instance session. This is very similar to what
we had up until now, except it is not a global object so forking is
possible as long as each child instantiates it's own client. The session
once created will be kept open for the duration of the client object
lifetime.

Please note: client instances are not thread safe. As can be seen from
above forking example - if you wish to use threading/multiprocessing,
you *must not* share client instances.

DocImpact

Related-bug: #1247056
Closes-Bug: #1297796
Co-authored-by: Nikola Dipanov <ndipanov@redhat.com>
Change-Id: Id59e48f61bb3f3c6223302355c849e1e99673410
Merge "Remove py3kcompat"
Merge "Print adminPass when rebuilding from shell"
Merge "Fix session handling in novaclient"