Add --filter-by option to server-list, network-list and flavor-list
management commands. The value of this option is a comma-seperated list
of key 'cond' val pairs and, when used, only the DB entries that satisfy all of
the specified conditions are displayed.

The exact keys that can be used are displayed in the help message. Also
when a wrong key is used, all the available keys are displayed to the
error message. No attempt is made to "sanitize" the keys.

Examples:
snf-manage server-list --filter-by "deleted=True,state=STARTED"
snf-manage server-list --filter-by userid=cstavr@grnet.gr,flavor__cpu=2
snf-manage flavor-list --filter-by "ram>=1024,disk<20"