https://fedorahosted.org/freeipa/ticket/1871

backup_and_replace_hostname() was doing three things:
    1. Given config file in 'key=value' style, replace value for a
       specified key (HOSTNAME)
    2. Backup original file and install a replacement
    3. Restore original security context after editing

We have several more places where parts of the functionality are needed,
thus making two tools in ipapython.ipautil:

    1. config_replace_variables(filepath, replacevars=dict(),
                                appendvars=dict())
       Replaces or appends values to specified keys, adding new key=value
       pairs if key was absent

    2. backup_config_and_replace_variables(fstore, filepath,
                                           replacevars=dict(),
                                           appendvars=dict())
       Backups config file and calls config_replace_variables()

A caller must handle security context after using these two tools.

In addition, as before, there is
ipapython.services.backup_and_replace_hostname() that uses
these common tools and restores security context after editing.

The code will be used extensively for systemd integration for Fedora 16.

Fixes:
    https://fedorahosted.org/freeipa/ticket/1871

https://fedorahosted.org/freeipa/ticket/1816

Refactor FreeIPA code to allow abstracting all calls to external processes and
dependencies on modification of system-wide configuration. A platform provider
would give its own implementation of those methods and FreeIPA would use it
based on what's built in packaging process.

https://fedorahosted.org/freeipa/ticket/1605