Verify pbkdf2 hash function implementation

[GabhenDM]

As discussed on #529 and #526, the api Dockerfile would throw the following error upon starting:

panic: crypto/hmac: hash generation function does not produce unique values

goroutine 1 [running]:
crypto/hmac.New(0xc0005cdbf0, 0xc0004d88d0, 0xf, 0x10, 0x0, 0x10)
	/usr/local/go/src/crypto/hmac/hmac.go:143 +0x370
golang.org/x/crypto/pbkdf2.Key(0xc0004d88d0, 0xf, 0x10, 0xc0003ee4c0, 0x40, 0x40, 0x186a0, 0x200, 0xc0005cdbf0, 0x0, ...)
	/go/pkg/mod/golang.org/x/[email protected]/pbkdf2/pbkdf2.go:43 +0x67
github.com/globocom/huskyCI/api/user.InsertDefaultUser(0xa28e60, 0xc00019c000)
	/go/src/github.com/globocom/huskyCI/api/user/user.go:58 +0x333
github.com/globocom/huskyCI/api/util/api.(*CheckUtils).checkDefaultUser(0xce2678, 0xc000022380, 0x98a011, 0x8)
	/go/src/github.com/globocom/huskyCI/api/util/api/api.go:139 +0x158
github.com/globocom/huskyCI/api/util/api.HuskyUtils.CheckHuskyRequirements(0xa33258, 0xce2678, 0xc000022380, 0x6, 0xb)
	/go/src/github.com/globocom/huskyCI/api/util/api/api.go:47 +0x22c
main.main()
	/go/src/github.com/globocom/huskyCI/api/server.go:46 +0x1ff

As found by @aranhams, that error validation was introduced in Go version 1.16, as on changelog:

    crypto/hmac

        New will now panic if separate calls to the hash generation function fail to return new values. Previously, the behavior was undefined and invalid outputs were sometimes generated.

We believe therefore that the implementation of the hash generation function was already previously with an error, and only begin to break after the validation was introduced in the crypto library.

As a workaround, we fixed the Golang version utilized in the API Dockerfile as 1.15, in order to allow normal execution of the project.

This issue is open in order to better investigate this error, and perform all necessary changes in order to be able to execute the API in Go's latest version without errors.