Do not eagerly check for existence in FilePayload

This is a TOCTOU violation and FilePayload.getInput already propagates
this.  This commit allows external callers like jclouds-cli to
introspect on the exception type, returning a more friendly error
message in some situations.
Expect exception from FilePayload.getInput

Previously the constructor threw this.  Fixes regression from ac58a70.
Cleanup of Azureblob code; as per pull request review. Additional items
not covered here are assigned specific bug ids.
Revert "Add Atmos-specific MD5 checksum header"

This reverts commit 98230e60a4e47c94ffe65f50470cf5ad51ca3f67.

This succeeded in my tests but integration tests fail.
Follow-up to Azureblob cleanup

See discussion at https://github.com/jclouds/jclouds/pull/66
Allow skipping duplicate-finder-plugin by setting skipDuplicateFinder property to true
JCLOUDS-195. Add egress firewall rules for CloudStack