Set nginx default settings for master node

- increase number of workers from 2 to half available CPUs.
- enable epoll by default
- enable tcpnopush
- enable tcpnodelay
- disable server_token

These settings allow to bootstrap many nodes in parallel allowing
anaconda and debian installer to get all files without timeout.

Partial-Bug: 1384510
Partial-Bug: 1349360

Change-Id: I44054cf2afa1fa5a70ba064f4957b964947b2b2c
Implements: blueprint 100-nodes-support
Signed-off-by: Sergii Golovatiuk <sgolovatiuk@mirantis.com>
Merge "Set nginx default settings for master node" into stable/5.1