Add implementation of command output post-processing

Backport of commit 3a982ff.

Add a doRegexExtOutputSub() function to mirror doRegExSub(). This
allows modukes ti apply an arbitrary regular expression
substitution to the output collected from external commands.

Signed-off-by: Bryn M. Reeves <bmr@redhat.com>
Obscure password in corosync-objctl output

The corosync-objctl command output may include fence device
passwords. Use a command output regex substitution to elide them
from generated reports.

Signed-off-by: Bryn M. Reeves <bmr@redhat.com>