Introduce a mysql_role resource for creating MySQL roles

[joestump]

What does this PR change?

• Adds a mysql_role resource that maps to CREATE ROLE. Fixes #46.
• Fixes the provider to work with MySQL 8.0.
• Introduces parallel Travis-CI tests that run tests against MySQL 8.0, 5.7, and 5.6.
• Adds role and roles arguments to mysql_grant to manage permissions on roles and to assign roles to users. Only works with MySQL 8+.

Proposed HCL

resource "mysql_role" "developer" {
  name = "developer"
}

resource "mysql_grant" "developer" {
  role       = "${mysql_role.developer.name}"
  database   = "${mysql_database.test.name}"
  privileges = ["SELECT", "UPDATE"]
}

resource "mysql_user" "jdoe" {
  user     = "jdoe"
  host     = "example.com"
}

resource "mysql_grant" "test" {
  user     = "${mysql_user.jdoe.user}"
  host     = "${mysql_user.jdoe.host}"
  database = "${mysql_database.test.name}"
  roles    = ["${mysql_role.developer.name}"]
}

TODO

• [x] Add tests to mysql_grant for role and roles attributes
• [x] Add documentation
• [x] Document that tls_option doesn't work for MySQL 8.0