Remove ResourceWarnings when loading AMIS and INSTANCE_TYPES
Remove ResourceWarnings when loading AMIS and INSTANCE_TYPES

When loading AMIS and INSTANCE_TYPES in moto.ec2.models a file handle is
potentially leaked when loading the JSON.  This results in a
ResourceWarning which is a bit of unnecessary noise.

Rather than pass a call to open() to json.load() this instead uses a
context-manager in a small private helper function.

This fixes https://github.com/spulec/moto/issues/2620

spulec/moto,william-richard/moto,william-richard/moto,spulec/moto,spulec/moto,william-richard/moto,spulec/moto,spulec/moto,william-richard/moto,spulec/moto,william-richard/moto,william-richard/moto