west_commands: sign: suppress imgtool prints

If `args.quiet` is set, suppress the useless `print` statements output
by `imgtool` (mcuboot script) by capturing `stdout`.

Old output:
```
[44/44] Linking C executable zephyr/zephyr.elf
Memory region         Used Size  Region Size  %age Used
           FLASH:      415192 B       824 KB     49.21%
             RAM:      163124 B       256 KB     62.23%
        IDT_LIST:          0 GB         2 KB      0.00%
image.py: sign the payload
image.py: sign the payload
```
New output:
```
[44/44] Linking C executable zephyr/zephyr.elf
Memory region         Used Size  Region Size  %age Used
           FLASH:      415192 B       824 KB     49.21%
             RAM:      163124 B       256 KB     62.23%
        IDT_LIST:          0 GB         2 KB      0.00%
```

Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>