Allow dgraph migrate with remote mysql server.

[arijitAD]

Fixes #4707
Adds flags to provide the IP address and port of the remote Mysql server.

Example:
dgraph migrate --config config.properties --output_schema schema.txt --output_data sql.rdf --host 10.191.30.251 --port 3306

config.properties content:
user = newuser
password = password
db = testDB

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This change is 

[mangalaman93]

Reviewable status: 0 of 2 files reviewed, 2 unresolved discussions (waiting on @arijitAD and @manishrjain)

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dgraph/cmd/migrate/run.go, line 61 at r1 (raw file):

	flag.StringP("separator", "p", ".", "The separator for constructing predicate names")
	flag.BoolP("quiet", "q", false, "Enable quiet mode to suppress the warning logs")
	flag.StringP("ip", "", "localhost", "The IP address of the database server.")

You could use h and p for shortnames for host and port.

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dgraph/cmd/migrate/run.go, line 100 at r1 (raw file):

	initDataTypes()

	pool, err := getPool(user, db, password, ip, port)

minor: the order of args should be ip, port, user, password, db or user, password, ip, port, db. It looks odd as of now.

[arijitAD]

Reviewable status: 0 of 2 files reviewed, 1 unresolved discussion (waiting on @mangalaman93 and @manishrjain)

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dgraph/cmd/migrate/run.go, line 61 at r1 (raw file):

Previously, mangalaman93 (Aman Mangal) wrote…

You could use h and p for shortnames for host and port.

p is already used by another flag.

[martinmr]

Reviewed 2 of 2 files at r2.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @arijitAD, @mangalaman93, and @manishrjain)

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dgraph/cmd/migrate/run.go, line 61 at r1 (raw file):

Previously, arijitAD (Arijit Das) wrote…

p is already used by another flag.

I'd call it host instead of IP since this supports hostnames as well as IP addresses.

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dgraph/cmd/migrate/run.go, line 72 at r2 (raw file):

	schemaOutput := conf.GetString("output_schema")
	dataOutput := conf.GetString("output_data")
	ip := conf.GetString("ip")

I'd also call this variable host instead of ip.

[arijitAD]

Reviewable status: 0 of 2 files reviewed, 2 unresolved discussions (waiting on @mangalaman93, @manishrjain, and @martinmr)

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dgraph/cmd/migrate/run.go, line 61 at r1 (raw file):

Previously, martinmr (Martin Martinez Rivera) wrote…

I'd call it host instead of IP since this supports hostnames as well as IP addresses.

Done.

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dgraph/cmd/migrate/run.go, line 72 at r2 (raw file):

Previously, martinmr (Martin Martinez Rivera) wrote…

I'd also call this variable host instead of ip.

Done.