Check if image already exists before doing anything else on GCE.

This fixes #1729.
Remove bucket_name from GCE config.

Fixes #1732. Forgot to do this in 23c947acf049b734c03ff3e24530f0310475b1a5.
Merge pull request #1738 from dcarlino/no_gce_bucket

Remove bucket_name from GCE config.
Merge pull request #1737 from dcarlino/failfast_gce

Check if image already exists before doing anything else on GCE.
CHANGELOG updates
packer: push configuration
command: push command
hook up push to commands
command/push: partially implemented, tests
command/push: close reader
command/push: actual upload to named build config
command/push: create the build config if we can
command/push: support custom endpoints
command/push: propagate token properly
command/push: works
command/push: if a token isn't given, don't set it
command/push: add base_dir setting
builder/amazon: inject special metadata for atlas artifacts
command/push: send artifact true/false if we're post-processing
command/push: warn for bad builds