Refactor: libpacemaker: Add an argument for tweaking attribute output.

There are various differences between the libpacemaker output and the
crm_attribute output:

• crm_attribute does not put double quotes around values.

• crm_attribute will output "value=(null)" for an empty value, while libpacemaker will use an empty string for the value.

Add an argument for controlling these differences in format. With this,
there should no longer be any differences between the attribute message
in the library and the one in crm_attribute.