Add command line flag to enable swift by default

The cloud-install shell scripts takes '-s' flag in order to enable swift from
installation. However, cloud-status takes '--enable-swift' if cloud-status
is invoked directly. It would be best to keep those flags similar, however,
the shell interpreter is limited w.r.t. getopts parsing.

Signed-off-by: Adam Stokes <adam.stokes@ubuntu.com>