Add support for role in location_prefers and location_add

[inouekazu]

This patch makes it easy to specify the role in location constraints.

# pcs constraint location -h

Usage: pcs constraint [constraints]...
    location <resource> [<role>] prefers <node>[=<score>] [<node>[=<score>]]...
    location <resource> [<role>] avoids <node>[=<score>] [<node>[=<score>]]...
    
    location add <id> <resource> <node> <score> [role=master|slave]
            [resource-discovery=<option>]