sap_hana_install: Further simplify checksum verification

[berndfinger]

We can modify the order of checksum verification to only perform the checksum calculation after verifying that a checksum for comparison is available, as follows:
1 - Check if a specific or global checksum file exists
2 - If yes, get the checksum from the checksum file. If there is no checksum, fail
3 - Only if there is a checksum entry for the file, calculate the checksum of the file to be checked.
4 - Assert that the two checksum matches

We don't have to include a enforce_checksum_verification variable to the file dict which is passed to verify_checksum.yml, because:
We perform a checksum verification if:
1 - the variable sap_hana_install_enforce_checksum_verification is set to yes, or
2 - if the variables for sapcar or sarfiles are not set, meaning that those files are auto-detected.
If the variable sap_hana_install_enforce_checksum_verification is not set or set to no, and if the variables for sapcar or for the sarfiles are set, then we skip the checksum verification at all.

[berndfinger]

Maybe it is better to never enforce checksum verification if the variable is not set, for example to simplify the installation of SAP HANA in test environments. If the files are already extracted, checksum verification outside of what hdblcm might be doing would not be performed anyway.

[berndfinger]

Solved in #69 .