Allow prefixing to be optional

The webhook supports the ability to prefix sources in R10K. However,
this ability is an all-or-none system. When prefixing is enabled, the
webhook will attempt to join a prefix to a branch name even when no
prefix is returned from the prefix_command. In some environments, there
may be the need to deploy at least one source without a prefix.

This commit adds the ability to make prefixing, when enabled, optional.
When the prefix_command does not return a value, R10K is invoked with
the branch name only.
Merge pull request #75 from trlinkin/optional_prefix

Allow prefixing to be optional