Allow the FreeBSD plugin to install without bash [GH-2485]

The default shell is "bash -l" which does not get installed by default
on FreeBSD. This change allows the plugin to override the default shell
and use a known installed shell (sh).
Merge pull request #2529 from markpeek/markpeek-freebsd-nobashshell

guests/freebsd: Allow the FreeBSD plugin to install without bash [GH-2485]