Implement BooleanArgument

BooleanArgument is different to FlagArgment because it accepts
both boolean values (True|False)
Change --private argument in project create/modify

In create: change argument from Flag to Boolean
In modify: add a --public Flag argument
In arguments: test BooleanArgument and fix minor bugs
Reflect astakos-related changes in documentation
Use iso8601 date format in account API requests
Add timezone support in DateArgument
Expect 200 in successful floating ip reassignment

Change expected value from 202 to 200 in reassign_floating_ip.