Add the version option to all Commands

Several Commands were missing the 'version' option. Add it to those
that were missing it.

Do not remove the version option before calling commands. This means
methods such as execute(), forward(), run() receive it.
Several of these needed `**options` added to their signatures.
Commands in the Cert plugin passed any unknown options to the underlying
functions, these are changed to pass what's needed explicitly.
Some commands in DNS and Batch plugins now pass version to commands
they call.

When the option is not given, fill it in automatically. (In a subsequent
commit, a warning will be added in this case).

Note that the public API did not change: all RPC calls already accepted
a version option. There's no need for an API version bump (even though
API.txt changes substantially).

Design page: http://freeipa.org/page/V3/Messages
Tickets:
  https://fedorahosted.org/freeipa/ticket/2732
  https://fedorahosted.org/freeipa/ticket/3294
Add ipalib.messages

The messages module contains message classes that can be added
to a RPC response to provide additional information or warnings.

This patch adds only the module with a single public message,
VersionMissing, and unit tests.
Since message classes are very similar to public errors, some
functionality and unit tests were shared.

Design page: http://freeipa.org/page/V3/Messages
Ticket: https://fedorahosted.org/freeipa/ticket/2732
Add client capabilities, enable messages

The API version the client sends can now be used to check what the client
expects or is capable of.

All version tests IPA does will be be named and listed in one module,
ipalib.capabilities, which includes a function to test a specific capability
against an API version.
Similarly to Python's __future__ module, capabilities.py also serves as
documentation of backwards-incompatible changes to the API.

The first capability to be defined is "messages". Recent enough clients can
accept a list of warnings or other info under the "messages" key in the
result dict.

If a JSON client does not send the API version, it is assumed this is a testing
client (e.g. curl from the command line). Such a client "has" all capabilities,
but it will always receive a warning mentioning that forward compatibility
is not guaranteed.
If a XML client does not send the API version, it is assumed it uses the API
version before capabilities were introduced. (This is to keep backwards
compatibility with clients containing bug https://fedorahosted.org/freeipa/ticket/3294)

Whenever a capability is added, the API version must be incremented.
To ensure that, capabilities are written to API.txt and checked by
`makeapi --validate`.

Design page: http://freeipa.org/page/V3/Messages
Ticket: https://fedorahosted.org/freeipa/ticket/2732
Rename the "messages" Output of the i18n_messages command to "texts"

This is to prevent a fatal name clash wih the new common "messages" Output.

Since i18n_messages is an internal plugin, the change does not affect
our public API.