Support unicode in resource refs

Unicode characters break bootstrap code:

2017-11-22 00:56:10,611 WARNING [-] Failed to register actions: Failed to register action "/opt/stackstorm/packs/testpack/actions/pythontest.meta.yaml" from pack "testpack": 'ascii' codec can't encode character u'\u2021' in position 10: ordinal not in range(128)
Traceback (most recent call last):
  File "./st2common/bin/st2-register-content", line 22, in <module>
    sys.exit(content_loader.main(sys.argv[1:]))
  File "/home/vagrant/st2/st2common/st2common/content/bootstrap.py", line 388, in main
    register_content()
  File "/home/vagrant/st2/st2common/st2common/content/bootstrap.py", line 338, in register_content
    register_actions()
  File "/home/vagrant/st2/st2common/st2common/content/bootstrap.py", line 219, in register_actions
    raise e
ValueError: Failed to register action "/opt/stackstorm/packs/testpack/actions/pythontest.meta.yaml" from pack "testpack": 'ascii' codec can't encode character u'\u2021' in position 10: ordinal not in range(128)

Should make sure this works for all resources, actions, rules, etc.

Start with st2common/content/bootstrap.py and the st2common/bin/st2-register-content entry point, following register_content() and register_actions() from the reported traceback. Reproduce the failure with the Unicode resource reference shown in the issue, then verify bootstrap registration works for actions, rules, and other resources without ASCII encoding errors.