cisco: Do not change supported_extension_aliases directly

cisco: Do not change supported_extension_aliases directly

cisco network plugin extends supported_extension_aliases in __init__
but supported_exntension_aliases is a class attribute and it is not
reset to the original even after each unit test finished.
To avoid this this patch copies supported_extension_aliases to
an instance attribute and extends it to ensure the class variable
is not changed. This reduces unnecessary logs in unit tests.

Change-Id: I3a7313f5ca2d10b1ae6ea961d9d05611aee055c0
Closes-Bug: #1292742