[rancid] Replace commands for specific hosts

[Valeriy Simonov]

For example, latest IOS versions of Cisco ASR1000 make tracelogs and a
generate lot of syslog messages when I enable archive log config. For now, I
couldn't found any solution for that (disabling log config is not an option
due to our policy). Also you have ignored case when specific ACL is changing
constantly. With current rancid, there are no escape from lots of reports
every period, and important information can be buried under all that
non-valuable changes.

As for line dependancy, I think that's can be a rare case, and after all it
should be a user responsibility. Suggested workflow is "make command
replace, test impact, leave satisfacted or repeat all above". Most paranoid
and foolproof solution can be a way to exclude specific commands for
specific hosts completely, but it has less flexibility. In my solution, we
can add "exclude .|^$" to scrub unwanted command output or change
unsupported command to some stub.

And we can add some logic to change parser function for command output, that
will give us more options.

Anyway, output filtering is a feature that I find unfortunately missing in
such a great tool as rancid.

I would argue that, as in the example below, if a device is changing
its bootflash every hour, then you likely have a bug in the operating
system.  The ASAs used to do that, but the bug was corrected in a
later revision of the OS.
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> While it may be easy to replace the command being sent to the device, what is the effect on the rancid code downstream?  While I cannot speak authoritatively about the contents of the source code, I do believe there are lines of code that are dependent on specific output strings from previous commands.
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