disable 'hg push' to TS repositories

[martenson]

deprecated in 15.10 https://docs.galaxyproject.org/en/master/releases/15.10_announce.html#deprecation-notices

results in

~/devel/hg/temp/asdsadsadas$ hg push
pushing to http://qqqqqq@<FQDN>/repos/qqqqqq/asdsadsadas
searching for changes
abort: HTTP Error 500: Pushing to Tool Shed is disabled.

This also adds a TS configuration option disable_push that defaults to True

part of #1928

[nsoranzo]

s/self.config[ 'disable_push' ]/self.config.get('disable_push', True)/

Should fix the traceback in https://jenkins.galaxyproject.org/job/docker-toolshed/1994/testReport/junit/tool_shed.functional.test_0310_hg_push_from_api/TestHgWebFeatures/test_0010_edit_and_commit/ :

File "/galaxy/lib/galaxy/webapps/tool_shed/framework/middleware/hg.py", line 78, in __call__
    if self.config[ 'disable_push' ]:
KeyError: 'disable_push'

[nsoranzo]

I've opened a PR against your branch in martenson#13 .

[martenson]

this is a WIP; I am writing more tests

[martenson]

reopened as #2044