Re-set common rsync and dsh parameters in mw-deployment-vars

This was first implemented in I702777b4a, but that change contained a bug and
had to be reverted (change I4e6675efe). The bug was that 'mwversioninuse' used
/bin/sh as the interpreter, which means it could not source
mw-deployment-vars.sh, as the latter file uses Bash arrays, which are not POSIX
compliant.

The only difference between this patch and I702777b4a is that /bin/bash is
specified as the interpreter in both 'mwversioninuse' and 'sync-common'.

Change-Id: I2800d01a6b29a323c33a4feeb44c55d60a612736