Fix panic when ip address without scheme is used for graphite backend

[johnewing1]

---

Description

Fixes panic when an IP address without a scheme was used as the graphite host. E.g.

[GraphiteConf]
    Host = "127.0.0.1:80"

Results in

2020/06/30 17:45:29 enabling syslog
2020/06/30 17:45:29 info: migrate.go:138: checking migrations
2020/06/30 17:45:29 info: search.go:208: Loading last datapoints from redis
2020/06/30 17:45:29 info: search.go:215: Done
2020/06/30 17:45:29 info: check.go:562: check alert example.graphite start with now set to 2020-06-30 16:45:29.095031
2020/06/30 17:45:29 info: check.go:562: check alert example.opentsdb.os.high.cpu start with now set to 2020-06-30 16:45:29.095031
2020/06/30 17:45:29 info: check.go:562: check alert example.opentsdb.cpu.lookup start with now set to 2020-06-30 16:45:29.095031
<nil>2020/06/30 17:45:29 error: expr.go:148: Error: runtime error: invalid memory address or nil pointer dereference. Origin: Schedule: Alert Name: example.graphite. Expression: avg(graphite("*.cpu.*.cpu.user", "5m", "", "host..cpu")) > 1, Stack: goroutine 92 [running]:
runtime/debug.Stack(0x2271b60, 0x2254880, 0x35f4ba0)
    /usr/local/Cellar/go/1.13.7/libexec/src/runtime/debug/stack.go:24 +0x9d
bosun.org/cmd/bosun/expr.errRecover(0xc000169e58, 0xc000290f20)

Type of change

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update

How has this been tested?

• [x] Unit test added which reproduces panic edge case.

Checklist:

• [x ] This contribution follows the project's code of conduct
• [x] This contribution follows the project's contributing guidelines
• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes
• [x] Any dependent changes have been merged and published in downstream modules