fetch2/svn: change 'rsh' parameter to 'ssh'

fetch2/svn: change 'rsh' parameter to 'ssh'

The functionality around the 'rsh' parameter seemed to be broken. The
'rsh' parameter was only used when 'protocol' was set to 'svn+ssh' which
is confusing. The 'rsh' parameter was used for setting the value of
'svn_RSH' environment variable, which however, is not supported by svn
(not at least according to SVN documentation).

This patch removes the 'rsh' parameter and replaces it with 'ssh'. This
new (optional) parameter is used when svn+ssh protocol is used and it
can be used to specify the ssh program used by svn. This is achieved by
setting the SVN_SSH environment variable which is mentioned in SVN
documentation.

Signed-off-by: Markus Lehtonen <markus.lehtonen@linux.intel.com>