Allow ipa-replica-conncheck and ipa-adtrust-install to read krb5 includedir

https://fedorahosted.org/freeipa/ticket/3132
Improve `ipa --help` output

Fix the usage string to match actual usage.

Add command description.

Put information about `ipa help topics` etc. to the epilog,
instead of using empty option groups. Use a custom formatter
to preserve newlines.

Add the -h/--help option manually to ensure consistent case
(capital S).

Part of the effort for https://fedorahosted.org/freeipa/ticket/3060
Print help to stderr on error

Whenever a command is used incorrectly, it should output
an error message (and possibly additional help) to stderr.

This patch adds a parameter to a bunch of places to allow
selecting either stdout or stderr for help output, and makes
badly called commands output to stderr only.

Part of the effort for https://fedorahosted.org/freeipa/ticket/3060
Store the OptionParser in the API, use it to print unified help messages

Make `ipa -h` and `ipa help` output the same message.

Since `ipa -h` output is generated by the OptionParser, we need to make
the parser available. Store it in `api.parser`.

Part of the effort for https://fedorahosted.org/freeipa/ticket/3060
Simplify `ipa help topics` output

This brings the output closer to `ipa help commands` and removes
extraneous information.

Part of the effort for https://fedorahosted.org/freeipa/ticket/3060
Add command summary to `ipa COMMAND --help` output

This makes the output identical to `ipa help COMMAND`.

Part of the effort for https://fedorahosted.org/freeipa/ticket/3060
Mention `ipa COMMAND --help` as the preferred way to get command help

This avoids the problem with ambiguous command/topic names.

No functionality is changed; `ipa help <COMMAND>` still works as before
if there's no topic with the same name.

https://fedorahosted.org/freeipa/ticket/3247
Parse command arguments before creating a context

This allows users to run `ipa COMMAND --help` even without
Kerberos credentials.

Part of the effort for https://fedorahosted.org/freeipa/ticket/3060
Add tests for the help command & --help options

Move the parser setup from bootstrap_with_global_options to bootstrap,
so all API objects have access to it.

Add some CLI tests for the help system.

Part of the effort for https://fedorahosted.org/freeipa/ticket/3060
In topic help text, mention how to get help for commands

This should prevent user confusion when topic help is requested
unintentionally, for example with `ipa help ping`.

See https://fedorahosted.org/freeipa/ticket/3247