Allow operators to use custom parameters with the ceilometer-upgrade
command. This is quite useful when using the dynamic pollster subsystem;
that sub-system provides flexibility to create and edit pollsters configs,
which affects gnocchi resource-type configurations. However, Ceilometer
uses default and hard-coded resource-type configurations; if one customizes
some of its default resource-types, he/she can get into trouble during
upgrades. Therefore, the only way to work around it is to use the
"--skip-gnocchi-resource-types" flag. This PR introduces a method for
operators to execute such customization, and many others if needed.

Depends-On: https://review.opendev.org/#/c/718190/
Change-Id: I92f0edba92c9e3707d89b3ff4033ac886b29cf6d