ovs-docker: Use a different mechanism to check for commands.

ovs-docker: Use a different mechanism to check for commands.

Currently, the script uses '--version' option from different commands
to check for their availability. uuidgen on Centos6 has been reported
not to have the '--version' option causing failure in script invocation.

This commit looks for the utilities in $PATH instead. The code is
copied from build-aux/dist-docs.

Reported-by: Michael J. Smalley <michaeljsmalley@gmail.com>
Suggested-by: Ben Pfaff <blp@nicira.com>
Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>