defaulting common_hostname to empty string

also extra vars overrides developer sandbox defaults
Merge pull request #1743 from edx/jarv/fix-flag-release

defaulting common_hostname to empty string
Make devpi optional for edxapp

To enable, set the `edxapp_devpi_enabled` variable to `true` and set
edxapp_pypi_local_mirror to point to wherever devpi is installed.
Add devpi_host setting to devpi role

Allow specifying a different host than localhost.
Allow restart handler to run when updating conf

Running the edxapp role with -t edxapp_cfg requires the app to be
restarted.
Add IONISX_AUTH to edxapp’s auth config
Allow custom AWS_S3_HOST
Add advanced problem types