Add field_labels parameter to cliutils.print_list

This patch allow setting custom heading for the resulting table.
This is required for cases, when the heading differs from field name
significantly, e.g. in Ironic client driver-list:
 hosts -> Active Host(s)

Change-Id: I02f41d2c0d5ab02856019b1cfc720a1136df9c2d
Closes-Bug: #1342391
Use MultiStrOpt for policy_dirs

We want to ensure that the values specified for directory names are
treated as strings, so use MultiStrOpt instead of the more general
ListOpt.

Change-Id: I46ab5d768b434133f681d9348ecb4c271d633c21
Merge "Use MultiStrOpt for policy_dirs"
Merge "Add field_labels parameter to cliutils.print_list"