[cloud] Run comprehensive AWS instance type tests

Define a representative list of AWS instance types, validate that this
list is comprehensive using aws-types, and test each instance type in
the list using aws-test.

Use an explicit region for each type within the list, rather than
relying upon the automatic region selection in aws-test.  Using an
explicit region allows types to be manually balanced across regions if
necessary to avoid hitting quota limits, and also allows an instance
type to be moved to a different region if transient conditions
(e.g. shortage of metal instance types) cause a false negative test
failure.

Allow for an explicit list of skipped instance types to be defined, so
that known-failing types may be omitted.  At the time of writing, only
the m8g.metal and m9g.metal families are known not to work.  There is
unfortunately no way to cause a matrix job to be skipped based on a
matrix value, so only the job name indicates that the job was skipped.

Signed-off-by: Michael Brown <mcb30@ipxe.org>