Fix checksum verification timing out on large OS images

[michalis1]

Fixes #410

Problem

get_remote_checksum hardcodes read_timeout=300 for the checksum command
(verify /md5 on IOS/ASA, run md5sum on IOS-XR), and nothing in the
verify_file -> compare_file_checksum -> get_remote_checksum chain accepts a
timeout. On Catalyst 9k hardware verify /md5 hashes at under 4 MB/s, so a
~1.2 GB image already exceeds 300s and current 2+ GB images need 500-700+
seconds. The result: file_copy / remote_file_copy transfers that completed
successfully on the device get reported as failures when the post-transfer
verification hits netmiko's absolute read timer. Retried jobs can also fail in
the pre-transfer verification, since the file is already on flash by then.