Removed undefined method in install_env.py file

There was a call from install_venv.py file to post_process method
of InstallVenv class, but this method is not defined in this class
as result it was raising an error. Even when this error doesn't
affect dependencies installation, it is not allowing to display user
information.

Change-Id: I499c0399eb961588d7e1491e1481412ffdda38b8
Closes-Bug: #1275025
Flavor ExtraSpecs containing '/' cannot be deleted

This change applies a regular expression in order to filter
flavor extraspects keys with invalid characters.
The characters allowed are: letters, numbers, underscores,
periods, colons, spaces and hyphens.
A new test flavor has been created which doesn't check the
keys in the post body. This flavor has been created in the
third place (instead of in the last) in order to keep
working existent test cases which depend on the last flavor
received in the get method.

Change-Id: Ifd86bed23a05a5946ae8b9ba6f6c9bf4b24b1d4c
Partial-Bug: #1256119
Merge "Flavor ExtraSpecs containing '/' cannot be deleted"