zpool-create.sh: allow features to be disabled

The zimport.sh script makes use of the zpool-create.sh script
to construct test pools for importing with older versions of
ZoL.  It is desirable to have a way to disable all the features
so new pools can be imported with older code.

The simplest and most flexible way to achieve this was to merge
the VERBOSE_FLAG and FORCE_FLAG in to a single ZPOOL_FLAGS
variable.  The contents of this variable will be used in the
'zpool create' allowing us to easily pass arbitrary flags.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>