Prepare for a reverse_http stager
Pymet reverse_http stager basic implementation
Patch pymeterp http settings
Escape inserted vars and fix core_loadlib
Pymeterp http proxy and user agent support
Fix pymeterp bugs from testing in osx and python3
Shorten the reverse_http stager by renaming a var
Add the test context to payloads_spec for py http
Fix two typos for payload specs
Fix os detection

See #4283
Fix os detection

See #3373
Land #4292, remove os_flavor usage.
Cosmetic tweaks for #4225
Python reverse_http stager, lands #4225
Merge master with merge of PR #4225