qa/vstart_runner: override adjust-ulimits, ceph-coverage & sudo

qa/vstart_runner: override adjust-ulimits, ceph-coverage & sudo

Overridding commands is much better than deleting these commands from
command argument string using Python since, unlike deleting, overridding
doesn't require parsing. A note has been added for this to
vstart_runner.py's module docstring and to Ceph Developer's Guide
document.

Since functions don't work with sh shell, to make overriding work
vstart_runner.py will use bash shell here onwards.

Signed-off-by: Rishabh Dave <ridave@redhat.com>