edgerouter.py: avoid python3 exception

edgerouter.py: avoid python3 exception

This fixes an error such as:

Exception: File "/master/poky/meta-yocto-bsp/lib/oeqa/controllers/edgeroutertarget.py", line 47
    self.image_fstype = self.get_image_fstype(d)
TabError: inconsistent use of tabs and spaces in indentation

(From meta-yocto rev: 5ff5dc2767dfef93c314e7465a52ca2dce553604)

(From meta-yocto rev: d1c50ea67f53b5fa2618b460efe4cc879d7cee0c)

Signed-off-by: Juro Bystricky <juro.bystricky@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Armin Kuster <akuster808@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>