Adding S3 snapshot location for elasticache

Only viable for redis engine
fix mismatched variable names in disk.create_snapshot
bump excon dependency
Merge pull request #2749 from ScotterC/elasticache-snapshot

Adding S3 snapshot location for elasticache
Merge pull request #2758 from erjohnso/master

[google|compute] fix mismatched variable names in disk.create_snapshot
defer fog-core deps to fog-core
bump fog-core dep
Fix a typo in error message that could cause problems for cut-n-pasters
Merge pull request #2762 from chrissnell/TypoFix

Fix a typo in error message that could cause problems for cut-n-pasters