Clean up activity environment variables

[quozl]

Ensure environment variables needed by Python activities are available in each of the three supported scenarios;

• started from Sugar,
• started from Terminal inside Sugar,
• started by other desktop environments.

Variables always available are;

SUGAR_ACTIVITY_ROOT
SUGAR_BUNDLE_ID
SUGAR_BUNDLE_NAME
SUGAR_BUNDLE_PATH
SUGAR_BUNDLE_VERSION

Variables also available when started from Sugar are;

SUGAR_ACTIVITIES_HIDDEN
SUGAR_APISOCKET_KEY
SUGAR_APISOCKET_PORT
SUGAR_GROUP_LABELS
SUGAR_HOME
SUGAR_MIME_DEFAULTS
SUGAR_PROFILE
SUGAR_SCALING
SUGAR_VERSION

Variables also available when started from Terminal are;

SUGAR_TERMINAL_VERSION

Other changes;

• use os.makedirs in place of distutils.dir_util.mkpath,
• avoid redundant setting of SUGAR_BUNDLE_PATH,
• do not set SUGAR_BUNDLE_ID unnecessarily,
• add explanatory comment,

Tested on Ubuntu 18.04.

Relates to discussion on https://github.com/sugarlabs/terminal-activity/pull/34