fencing: Add verbose_level option

Currently, verbose is a boolean, all-or-nothing option. It would be
useful to be able to set more granular verbosity levels when debugging
an issue.

This patch adds a verbose_level option to allow setting higher
verbosities. This option can be set explicitly (e.g.,
--verbose-level=3). Alternatively, multiple verbose flags (e.g.,
-vvv) can be passed on the command line; verbose_level is set to
the number of -v flags.

If both an explicit verbose_level and one or more -v options are
set, the verbose_level takes precedence and overrides any -v flags.
If verbose_level > 0 and there are no -v flags, the verbose
option is enabled implicitly.

Later patches will enable individual fence agents to use the new
verbosity levels.

Related to: RHBZ#1853973