Faster nova fixed key generation

Using bc 64 times in loop is too verbose and slow,
replacing the echo/bc loop with hexdump and urandom.

The hexdump approach is 75 times faster and
does not floods the debug logs.

Using the common function for generating,
this kind of string with lib/heat and by the read_password.

Change-Id: If6a86dfaf0c21e2635c6de0a7b96a8ed7ec5b507
Merge "Faster nova fixed key generation"