APPSRE-7520: Retry function clean-up and refactor

[kwilczynski]

Clean-up and refactor the Retry function so that:

• Use errors.As() instead of a type assertion to ensure that wrapped errors work correctly
• Refactor the jitter() function to use a more secure random number generator from the crypto/rand package
• Factor out some magic values into constants, such as the sleep step multiplier and jitter divider
• Drop surplus if-else around the login() function and return the error value directly

Signed-off-by: Krzysztof Wilczyński kwilczynski@redhat.com