Refactor tempest config generator

Use `requests` library instead of `urllib2` & `httplib`
Main profit from `requests` library:
- `requests` is HTTP for Humans;
- `requests` is well documented[1];
- `requests` can save downloaded files by parts, so usage of memory
is less(useful when downloading big data, for example - some images)
- `requests` has same name as in Python 2x and Python 3x

Use `inspect.getmembers`[2][3] for discovery of each function for section
generator. Profit : decrease lines of code

Add checks for existence of subnet in 'network' section.

Move function `_write_config`[4] from class `Tempest`[5] to `TempestConf`.
Reason: this function not related to verifier for Tempest.

Fix message for exception when user doesn't have admin role.

Move name of section into kwargs to each section function.
Profit: less string objects with section names

[1] - http://docs.python-requests.org/en/latest/
[2] - https://docs.python.org/2/library/inspect.html#types-and-members
[3] - https://docs.python.org/3.3/library/inspect.html#types-and-members
[4] - http://git.io/E1zPOQ
[5] - http://git.io/8Ru-Yw

Change-Id: I8307ec33dd93ef055450f58d7cd55bf6b200f249