Rax mod utils improvements

Add rax_keypair module

Add ability to specify a VirtualIP ID for sharing a VIP with another LB

Add hints about what data goes into the variable

If a executable is defined but not found the module should fail.

If a executable is defined but not found the module should fail. Addresses https://github.com/ansible/ansible/issues/5781.

Added info on --skip-tags to tags documentation.

Using dict(k=v) means the key was always named 'k', as the variable
is not interpreted there.

Fixes #5801

Mention shell module in command documentation

Update wait_for documentation - remove extra quote

Fixed typo

Used the twice in a row

Fix broken Tower links

Also note AWX is named "Tower" now, as we found that simpler.

In particular, do not rely on the $USER environment variable always existing.
tmux for example seems to clear it, causing lots of invalid messages:
"previous known host file not found"

This broke in commit 80fd22dc, but instead of reverting that commit, we now
fall back to expanding just ~ when $USER is not set.

be permissive with BSD with service enabled upper/lower/mixed cases

Only specify to USE_KEYRING as the api_key/password when we actually got a keyring_username from ~/.pyrax.cfg