Abstract OS log names computation

The various OS operations create log files in a specific directory
(constants.LOG_OS_DIR). The construction of the log names is however
spread and duplicated across multiple functions.

This patch abstracts this into a separate function that also validates
the log name, which should be safer in the long run. We also rename the
export logs from having a prefix of “exp” to “export”, since it was the
only operation that had an abbreviated prefix.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>