Handle yaml unsafe loader warning

[dyokelson]

The yaml warning displays on every system init command (but not experiment init)

Example:

$ ./bin/benchpark system init --dest=ruby-system cts cluster=ruby
Hold tight, Benchpark is bootstrapping itself.
Hold tight, Benchpark is bootstrapping itself.
Hold tight, Benchpark is bootstrapping itself.
Hold tight, Benchpark is bootstrapping itself.
Hold tight, Benchpark is bootstrapping itself.
/g/g19/brink2/.benchpark/ramble/lib/ramble/ramble/repository.py:1006: UnsafeLoaderWarning:
The default 'Loader' for 'load(stream)' without further arguments can be unsafe.
Use 'load(stream, Loader=ruamel.yaml.Loader)' explicitly if that is OK.
Alternatively include the following in your code:

import warnings
warnings.simplefilter('ignore', ruamel.yaml.error.UnsafeLoaderWarning)

In most other cases you should consider using 'safe_load(stream)'
yaml_data = yaml.load(reponame_file)