LU-11634 tests: sanityn/test_77 improvements

LU-11634 tests: sanityn/test_77 improvements

sshd limits number of simultaneous unauthenticated connections via
MaxStartups configuration parameter. By default, 10 connections are
allowed. nrs_write_read() tries to run up to 32 do_nodes() in
parallel, causing sshd to drop some of connections.

The fix is to have do_nodes() to start required number of dd-s in
parallel.

Minor changes which were probably meant at the development:
- Test filenames include $HOSTNAME so that each client worked with its
own file, it seems. Add missing escaping backslashes so that $HOSTNAME
worked as expected.
- Add conv=notrunc parameter for dd-s which write lustre file at
  different seeks.
- Have reading dd-s to read files which were especially created for
  that.
- use /dev/null instead on /dev/zero to throw read data away.

Signed-off-by: Vladimir Saveliev <c17830@cray.com>
Change-Id: I496b0f6b50811351ac8e0e606cf5a20843fab5d4
Cray-bug-id: LUS-2493
Test-Parameters: testlist=sanityn envdefinitions=ONLY=77
Reviewed-on: https://review.whamcloud.com/33607
Reviewed-by: Elena Gryaznova <c17455@cray.com>
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andrew Perepechko <c17827@cray.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>