scripts: ShellCheck found cleanups

Checked all the bash scripts with

 shellcheck --shell=bash

checking with and without the following exclude list

 --exclude=SC1091,SC2004,SC2006,SC2086,SC2143,SC2155

Even with the exclude list there were some warnings not fixed with
this patch, but those are OK.

Signed-off-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>