Simplifies argparse for `--force` in securedrop-admin
Simplifies argparse for `--force` in securedrop-admin

Based on review by @fowlslegs in #1881. According to the argparse docs
[0], there's a shorthand approach for when flags toggle boolean values.
Also adds a shorthand `-f` option since that feels intuitive.

[0] https://docs.python.org/2/library/argparse.html#action

garrettr/securedrop,heartsucker/securedrop,garrettr/securedrop,micahflee/securedrop,heartsucker/securedrop,micahflee/securedrop,conorsch/securedrop,conorsch/securedrop,garrettr/securedrop,ehartsuyker/securedrop,ehartsuyker/securedrop,micahflee/securedrop,conorsch/securedrop,garrettr/securedrop,heartsucker/securedrop,conorsch/securedrop,ehartsuyker/securedrop,ehartsuyker/securedrop,ehartsuyker/securedrop,heartsucker/securedrop,micahflee/securedrop,ehartsuyker/securedrop,conorsch/securedrop,heartsucker/securedrop