GuestOS: Support the os-release standard

GuestOS: Support the os-release standard

While bringing up Amazon Linux as a guest, it was discovered that
Amazon Linux was not LSB compilant. The LSB standard defines a way
to uniquely determine which distro one is running on. Since our code
was based on the LSB standard, we made some changes to explicitly
detect Amazon Linux and handle it as a special case.

Amazon let us know that there is a "new" standard - os-release - which
they are compliant with. One must explicitly install Amazon Linux in a
special way to obtain the LSB compliance.

Researching the "new" standard, it turned out to not be that different
than the LSB standard. Further research showed that some distros (e.g.
Ubuntu) are releasing with both standards.

This change refactors the existing code, removes the explicit Amazon
Linux checking, and accepts the os-release standard. Amazon Linux then
"just falls out".