Locale should be set on st2-docker environment

[jbrownsc]

Description
Some Stack Storm commands use the system locale to encode values printed to the screen (st2 run --tail for example). The LC_ALL environment variable can be set to whatever locale the system supports and Stack Storm will honor that encoding.

See StackStorm/st2#4120 (comment) for more information

Steps to reproduce
Good:

root@c3c4ea471142:/opt/stackstorm# LC_ALL=en_US.utf-8 st2 run --tail core.local cmd="printf '\xE2\x98\xA0'"
Tailing execution "5af482e73ce5fe4ce3623747"
Execution 5af482e73ce5fe4ce3623747 has started.

☠
Execution 5af482e73ce5fe4ce3623747 has completed (status=succeeded).

id: 5af482e73ce5fe4ce3623747
status: succeeded
parameters:
  cmd: printf '�'
result:
  failed: false
  return_code: 0
  stderr: ''
  stdout: "☠"
  succeeded: true

Bad:

root@c3c4ea471142:/opt/stackstorm# st2 run --tail core.local cmd="printf '\xE2\x98\xA0'"
Tailing execution "5af483d93ce5fe4ce362374a"
Execution 5af483d93ce5fe4ce362374a has started.

ERROR: 'ascii' codec can't encode character u'\u2620' in position 0: ordinal not in range(128)